Instructions

• Preheat the oven to 350°F.
• In a food processor (or in a bowl using a pastry cutter or two forks), combine 1 ½ cups all-purpose flour, 3 tablespoons unsweetened cocoa powder, ¼ cup granulated sugar, and ¼ teaspoon kosher salt; pulse or mix until evenly combined.
• Add 1 ½ sticks (12 tablespoons) chilled unsalted butter and pulse or cut in until the mixture is crumbly and the butter pieces are about the size of small peas.
• Add 1 egg and ¾ teaspoon vanilla extract and pulse or mix just until incorporated; the dough should still be slightly crumbly (do not overwork into a smooth ball).
• Line the bottom of an 8-inch springform pan with parchment paper and lock the side in place. Transfer the dough to the pan and press it evenly across the base. Prick the crust all over with a fork.
• Bake the crust for 20 minutes. Transfer the pan to a rack and let the crust cool completely in the pan, about 30 minutes.
• Once cooled, remove the side of the springform pan, peel the parchment paper away from under the crust, and then reattach the side of the pan.
• Reserve 3 tablespoons from the 1 ½ cups heavy cream for the gelatin. Sprinkle 1 teaspoon unflavored gelatin evenly over those 3 tablespoons of cold cream in a small heatproof bowl or small saucepan and let stand 5 minutes to bloom.
• Gently heat the gelatin-and-cream mixture over low heat, stirring, until the gelatin is melted and the mixture is warm. Do not boil (about 1–2 minutes).
• Whisk ½ cup chocolate hazelnut spread into the warm gelatin mixture until smooth, then remove from heat.
• In a large bowl, whisk ½ cup mascarpone cheese with the chocolate-hazelnut-gelatin mixture until smooth and fully combined.
• In a separate large, chilled bowl, combine the remaining heavy cream (the rest of the 1 ½ cups after reserving 3 tablespoons), 2 tablespoons unsweetened cocoa powder, and 3 tablespoons granulated sugar. Beat starting at low speed until combined, then increase speed to high and beat until the cream holds soft peaks.
• Whisk one-third of the whipped cream into the mascarpone mixture to lighten it, then gently fold in the remaining whipped cream until homogeneous and smooth.
• Spoon the mousse filling onto the prepared crust in the springform pan and smooth the top with a spatula. Cover and chill, refrigerated, for at least 3 hours (or until set).
• To make the ganache, place ⅕ cup heavy cream plus 1 tablespoon heavy cream in a small saucepan and bring just to a simmer, then remove from heat. Add 7 ounces chopped bittersweet chocolate, let stand 1 minute, then whisk until smooth.
• Transfer the ganache to a bowl and let it cool, stirring occasionally, until it is thickened but still pourable, about 20 minutes.
• Run a warm thin knife around the inside edge of the springform pan, then remove the side. Slide the cake off the pan base onto a serving plate.
• Pour the ganache over the top of the cake and spread gently, allowing excess to drip down the sides. Chill briefly if needed to set the ganache, then serve.