Ingredients
For shortbread base
* 2 tablespoons hazelnuts, toasted and skins rubbed off
* 3 tablespoons sugar
* 1/2 cup all-purpose flour
* 1/2 stick (1/4 cup) unsalted butter, softened
* 2 tablespoons unsweetened Dutch-process cocoa powder
* 1/8 teaspoon salt
For mousse
* 1 teaspoon unflavored gelatin (from a 1/4-oz envelope)
* 3 tablespoons cold water
* 1/2 cup chocolate hazelnut spread such as Nutella (5 oz)
* 1/2 cup mascarpone (1/4 lb)
* 1 1/2 cups chilled heavy cream
* 2 tablespoons unsweetened Dutch-process cocoa powder
* 3 tablespoons sugar
For ganache
* 1/4 cup plus 1 tablespoon heavy cream
* 3 1/2 oz fine-quality bittersweet chocolate (not unsweetened), chopped
* Special equipment: an 8-inch (20-cm) springform pan; parchment paper