For the Torte:
3/4 cup skinned hazelnuts, lightly toasted
9 ounces bittersweet chocolate, finely chopped
9 ounces milk chocolate, finely chopped
3 tablespoons unsalted butter, plus more for greasing pan
1 1/4 cups Nutella
6 large eggs
1/3 cup granulated sugar
1/8 teaspoon salt
1/4 cup heavy cream, cold

For the Garnish:
2 ounces bittersweet chocolate, finely chopped
16 whole skinned hazelnuts
1/4 cup heavy cream
Cocoa for dusting cake