2 cups sugar
1 3/4 cups all-purpose flour, plus for dusting
3/4 cup cocoa powder
1 1/2 teaspoons baking powder
1 1/2 teaspoons baking soda
1 teaspoon salt
1 cup milk
1/2 cup vegetable oil
2 teaspoons vanilla extract
1 cup boiling water
Lemon juice, for wiping out bowl
10 large egg whites
2 1/2 cups sugar
3 cups (6 sticks) unsalted butter, cut into cubes, plus for greasing cake pans
1 tablespoon plus 1 teaspoon pure vanilla extract
3 tablespoons hazelnut spread
1/2 cup chopped toffee crunch bar
For the cake: Preheat the oven to 350 degrees F. Grease and flour two 9-inch round cake pans.
Stir together the sugar, flour, cocoa, baking powder, baking soda and salt in a stand mixer bowl. Add the eggs, milk, oil and vanilla; beat on medium speed for 2 minutes. Stir in the boiling water; the batter will be thin. Pour the batter into the prepared cake pans.
Bake for 30 to 35 minutes, or until a wooden pick inserted in center comes out clean. Cool for 10 minutes, and then unmold the cakes onto wire racks and let cool completely.
For the frosting: Wipe out the mixer bowl with paper towels and lemon juice to remove any trace of grease. Add the egg whites and sugar, and heat over a pot of simmering water, whisking constantly but gently, until the mixture reaches 140 degrees F on a candy thermometer. (If you don't have a candy thermometer, the sugar should be completely dissolved and the egg whites hot.)
Transfer the mixer bowl to the stand mixer and whip on high speed, using the whisk attachment, until the meringue is white, thick and glossy, and holds stiff peaks, 8 to 10 minutes.
Switch over to the paddle attachment and, with the mixer on low speed, add the butter cubes, one at a time, mixing until fully incorporated and smooth. Add the vanilla, salt and hazelnut spread. (Makes 10 cups.)
To assemble: Level and tort both cakes--you will now have 4 layers. Place one cake layer on a cake board, spread with a generous layer (about 1/2 inch) of the hazelnut buttercream, and sprinkle the toffee crunch on top. Repeat until you have stacked all 4 cake layers. Frost the cake with the remaining buttercream. Slice and enjoy.