1 ½ cups unsalted butter, room temperature
1 ½ cups superfine sugar
3 teaspoons instant espresso dissolved in ½ cup boiling water
6 eggs
2 tablespoons kahlua
2 tablespoons whole milk
3 cups all-purpose flour
2 tablespoons baking powder
1/3 cup cocoa powder
1 teaspoon kosher salt

For the Frosting:

12 ounces semisweet chocolate
12 ounces bittersweet chocolate
5 large egg whites
1 ½ cups superfine sugar
1 pinch cream of tartar
2 teaspoons kosher salt
1 pound unsalted butter, room temperature
2 teaspoons hazelnut extract
1 cup lightly toasted hazelnuts, coarsely chopped