Twelve Layer Mocha Cake

Name: Twelve-Layer Mocha Cake
Ingredients: 4 large egg yolks at room temperature 30 minutes; 2 tablespoons whole milk; 1/2 teaspoon pure vanilla extract; 3/4 cup sugar, divided; 1/2 cup all-purpose flour; 1/4 teaspoon salt; 2 large egg whites at room temperature 30 minutes; 6 ounces fine-quality 60%-cacao bittersweet chocolate, finely chopped; 1/4 cup water; 5 large eggs, separated, at room temperature 30 minutes; 1/2 cup sugar, divided; 1 tablespoon unsweetened cocoa powder; 2/3 cup hazelnuts (3 1/2 ounces); 3 large egg whites at room temperature 30 minutes; 1/8 teaspoon salt; 1/8 teaspoon cream of tartar; 1/2 cup sugar; 1/3 cup water; 2 tablespoons sugar; 1 teaspoon instant-espresso powder; Coffee and mocha buttercreams; Equipment: 3 (15-by 10-inch) 4-sided sheet pans (1/2 inch deep)