INSTRUCTIONS
1. FOR THE CAKE: Adjust oven rack to middle position and heat oven to 350 degrees. Grease three 8-inch round cake pans, line with parchment paper, then grease and flour pans. Process 1 ½ cups walnuts in food processor until finely chopped, about 1 minute. Prepare yellow layer cake batter according to directions, stirring in ground walnuts until just combined. Scrape equal amounts of batter into prepared pans and bake until toothpick inserted in center comes out clean, about 25 minutes. Cool cakes in pans on wire rack for 10 minutes. Remove cakes from pans, discarding parchment, and cool completely, about 2 hours.
2. FOR THE CANDIED NUTS: Line rimmed baking sheet with aluminum foil and set wire rack in sheet. Simmer ¼ cup syrup in small saucepan over low heat until reduced by half, 2 to 3 minutes. Stir remaining ½ cup walnuts into syrup, then pour onto wire rack. Let sit until coating is firm, about 10 minutes.
3. FOR THE FROSTING: Whisk sugar, remaining ½ cup syrup, egg whites, and extract together in bowl and place over medium saucepan with ½ inch of barely simmering water. Using hand mixer, whip mixture on medium-high speed until stiff peaks form, 6 to 8 minutes. Remove from heat and whip until mixture has cooled and is very thick and glossy, 8 to 10 minutes.
4. TO ASSEMBLE: Place 1 cake round on serving platter. Spread 1 cup frosting over cake, then top with second cake round. Spread another 1 cup frosting over cake, then top with third cake round. Spread remaining frosting evenly over top and sides of cake. Garnish with candied walnuts. Serve.
