Walnut brownie cake:
1 cup sifted cake flour (not self-rising)
1 teaspoon baking soda
1/4 teaspoon salt
2 cups packed light brown sugar
12 tablespoons (1 1/2 sticks) unsalted butter, cut into tablespoons
1/4 cup water
6 ounces unsweetened chocolate, coarsely chopped
3 large eggs, at room temperature
2 teaspoons vanilla extract
1 1/2 cups walnuts, finely chopped

White chocolate mousse:
1/4 cup water
1 envelope unflavored powdered gelatin
1/4 cup cornstarch, lightly spooned into measuring cup and leveled with the straight edge of a knife
2 cups milk, divided
2 large eggs plus 1 large egg yolk
1/3 cup granulated sugar
6 ounces Swiss white chocolate, finely chopped
2 tablespoons unsalted butter, cut into 1/2-inch cubes
2 teaspoons vanilla extract
1 3/4 cups heavy (whipping) cream

White and dark chocolate triangles:
5 ounces Swiss white chocolate, coarsely chopped
5 ounces Swiss dark chocolate, coarsely chopped

Dark chocolate curls:
One 6-ounce chunk dark chocolate, about 3/4-inch thick

Make the walnut brownie cake:
1. Position a rack in the center of the oven and preheat to 350 degrees F. Lightly butter the bottoms and sides of two 9-by-1 1/2 inch round cake pans. Line the bottoms with circles of baking parchment or waxed paper. Dust the sides of the pans with flour and tap out the excess.
2. In a medium bowl, using a wire whisk, stir together the flour, baking soda and salt, until thoroughly blended. Sift the flour mixture onto a piece of
waxed paper.
3. In a large saucepan, combine the sugar, butter and water. Cook, stirring occasionally over medium heat until smooth. Continue cooking until the sugar mixture comes to a boil. Remove the pan from the heat. Add the chocolate to the saucepan. Let the mixture stand for thirty seconds to melt the chocolate. Stir until smooth. Transfer the chocolate mixture to a large bowl and cool for 10 to 15 minutes, until tepid.
4. Using a wire whisk, stir in the eggs one at a time, until smooth. Stir in the vanilla. Using a rubber spatula, fold in the flour until combined. Stir in the walnuts until blended.
5. Divide the batter evenly between the prepared pans and smooth it with a spatula. Bake the cake layers for 15 to 20 minutes, until a toothpick inserted two inches from the edge of the pan comes out with a few moist crumbs clinging to it.
6. Cool the cake layers in the pans set on wire racks for 15 minutes. Run a thin-bladed knife around the edges of the cake layers to loosen them. Invert them onto wire racks. Peel off the paper circles and leave them loosely set so that they are right side up. Cool the cake layers completely.

Make the white chocolate mousse:
7. Put the water in a small heatproof cup. Sprinkle the gelatin over the water and let the mixture soften while you prepare the mousse.
8. In a medium bowl, whisk the cornstarch and 1/2 cup of the milk until smooth. Whisk in the eggs and egg yolk until blended.
9. In a heavy, medium, noncorrosive saucepan, combine the remaining 1 1/2 cups milk and sugar. Cook over medium heat, stirring occasionally, until small bubbles form around the side of the pan . Gradually whisk 1 cup of the hot milk into the cornstarch/egg mixture until blended. Pour this mixture back
into the saucepan.
10. Continue cooking over medium heat, whisking constantly until the mixture starts to form bubbles. Continue stirring and boil the mixture for 2 minutes. Take the pan off the heat. Add the chocolate and gelatin mixture. Whisk until smooth. Stir in the butter until blended.
11. Strain the mixture through a sieve into a large bowl. Stir in the vanilla. Place the bowl over a bowl of iced water and stir for 5 to 10 minutes, until cool. Remove the white chocolate mixture from the bowl of iced water.
12. In a chilled large bowl, using a handheld electric mixer set at medium speed, beat the cream until soft peaks begin to form. Do not overwhip the cream. Using a rubber spatula, fold one-third of the whipped cream into the white chocolate mixture to lighten it. Fold in the remaining whipped cream.

Assemble the cake:
13. Remove the side from a 9-by-3-inch springform pan. Trim a 10-inch cardboard cake circle so that it fits snugly within the curved lip of the bottom of the springform pan. Reattach the side of the springform pan. Remove the paper from the bottom of the cake layers. Place one of the cake layers into
the prepared pan.
14. Scrape 3 cups of the white chocolate mousse over the cake layer in the pan. Using a small offset metal cake spatula, spread the mousse in an even layer over the cake. Make sure that the mousse is evenly distributed within
the narrow space between the side of the pan and the edge of the cake layer. Place the second cake layer over the mousse. Scrape 1 1/2 cups of the remaining white chocolate mousse or enough mousse to fill the space between the top layer of cake and the top edge of the pan. Spread the mousse in an even layer over the cake using the rim of the pan as a guide. With a long metal cake spatula, scrape off any excess mousse. Transfer the remaining white chocolate mousse to a small bowl and cover with plastic wrap. Refrigerate the assembled cake and reserved mousse for at least 4 hours, or overnight.

Make the chocolate triangles:
15. Separately melt the white and dark chocolates according to the melting instructions in the Chocolate Key. Line two flat baking sheets with uncreased pieces of aluminum foil. Draw a 9-inch circle on each piece of foil. Tape the four corners of each piece of foil to the baking sheets to hold it in place.
16. Pour the melted white chocolate into the center of a traced circle on one of the foil-lined pans. Using a small offset metal cake spatula, spread the chocolate evenly until it fills in the circle. Refrigerate the baking sheet for 5
to 10 minutes, until the chocolate feels firm, but not brittle.
17. With a long sharp knife, cut the circle into 12 wedges. Using an offset metal cake spatula, gently lift the wedges off the foil and transfer them to a waxed paper-lined baking sheet. If the wedges become soft while you are cutting them, return the baking sheet to the refrigerator briefly until the chocolate is firm again. You will have 4 extra triangles in case of breakage.
18. Repeat the above two steps to make 12 triangles using the melted dark chocolate and the second foil-lined baking sheet. Before making the dark chocolate triangles, spoon about 1/2 tablespoon of the dark chocolate onto the foil next to the traced 9-inch circle. Spread the chocolate into a small circle that is about 1/16-inch thick. When the chocolate is firm, use a 3/4-inch round cutter to make a circle to put on the top center of the cake. Store the white and dark chocolate triangles and the circle in the refrigerator until you are ready to decorate the cake.

Make the chocolate curls:
19. Warm the 6-ounce chunk of dark chocolate by placing it on a piece of waxed paper. Microwave on MEDIUM (50 percent) power for 10-second intervals, until the chocolate starts to soften slightly. This can take 3 to 6 tries, depending on the wattage of the microwave oven, type and brand of chocolate and the temperature of the kitchen. The chocolate should soften slightly, not melt. Alternatively, soften the chocolate by heating it with a portable blow dryer. Turn the chocolate chunk every few seconds, until it begins to soften.
20. Line a baking sheet with waxed paper. Grip the chocolate chunk with a folded paper towel so your hand does not melt the chocolate. Using a sharp vegetable peeler, scrape one narrow side of the chocolate chunk in a downward motion, forming loosely shaped curls. As you form the curls, let them fall onto the waxed paper. Refrigerate the curls on the baking sheet until you are ready to decorate the cake.

Unmold the cake:
21. Using a portable blow dryer or damp (not wet) very hot towel, carefully heat the outside of the springform pan until the edge of the mousse melts slightly. Release the clamp on the side of the springform pan and gently lift it up and remove it from around the side of the cake.

Decorate the cake:
22. Reserve about 3/4 cup of the chilled white chocolate mousse for piping on the top of the cake. Using an offset metal cake spatula, spread a thin layer of the remaining white chocolate mousse around the side of the cake to coat the exposed cake layers.
23. Lift up the cake on the cardboard cake circle, supporting the bottom of the cake in one hand. Use a metal spatula to lift up the chocolate curls and press them against the side of the cake, working around the cake until the side is completely covered with curls. Put the cake on a serving plate.
24. Fill a pastry bag fitted with a large closed star tip (such as Ateco #4) with the reserved 3/4 cup of white chocolate mousse. Pipe 1 rosette in the top center of the cake and 16 rosettes evenly spaced around the top outer edge of the cake.
25. Alternate 16 dark and white triangles around the top of the cake. Place each triangle so that the widest portion rests at a slight angle on a piped rosette at the edge of the cake. Place the chocolate circle in the middle of the cake so that it covers the points of the triangles and the single piped rosette. Refrigerate until ready to serve.

 
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