Chocolate-Glazed Hazelnut Mousse Cake

INGREDIENTS

• 1/2 stick (1/4 cup) unsalted butter, softened
• 1/2 cup mascarpone (1/4 lb)
• 1/2 cup chocolate hazelnut spread such as Nutella (5 oz)
• 1 teaspoon unflavored gelatin (from a 1/4-oz envelope)
• 2 tablespoons unsweetened Dutch-process cocoa powder
• 1 1/2 cups chilled heavy cream
• 1/8 teaspoon salt
• 2 tablespoons hazelnuts, and skins rubbed off
• 3 tablespoons cold water
• 1/2 cup all-purpose flour
• 3 tablespoons sugar
• 1/4 cup plus 1 tablespoon heavy cream
• 3 1/2 oz fine-quality bittersweet chocolate (not unsweetened), chopped