For cake layers:

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

For soufflélayers:

6 ounces fine-quality 60%-cacao bittersweet chocolate, finely chopped
1/4 cup water
5 large eggs, separated, at room temperature 30 minutes
1/4 teaspoon salt
1/2 cup sugar, divided
1 tablespoon unsweetened cocoa powder

For meringue layers:

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

For syrup:

1/3 cup water
2 tablespoons sugar
1 teaspoon instant-espresso powder

For filling:

Coffee and mocha buttercreams

Equipment: 3 (15-by 10-inch) 4-sided sheet pans (1/2 inch deep)