Hazelnut Mousse Cake with Chocolate Glaze

For the shortbread base: Position your oven rack in the middle level and preheat it to 350 degrees Fahrenheit. Invert the bottom of your springform pan, making it easier to detach the shortbread base later. Securely lock the side of the pan and line the bottom with a round piece of parchment paper.

Combine hazelnuts and sugar in a food processor and pulse until the nuts are finely ground. Add flour, butter, cocoa powder, and salt to the processor and pulse until a dough forms.

Press the dough evenly onto the bottom of your springform pan using your fingers. Use a fork to prick all over the dough, then bake until it is just dry to the touch, approximately 18-20 minutes. Transfer the cooled shortbread base from the pan to a wire rack and let it cool completely, about 30 minutes. Remove the side of the pan and carefully detach the parchment paper from underneath the shortbread base, then reattach the side of the pan around the shortbread base.

While the shortbread cools, prepare the mousse: Sprinkle gelatin over water in a 1- to 1 1/2-quart heavy saucepan and let it soften for about 5 minutes. Heat the gelatin mixture over low heat, stirring constantly, until it is just melted, about 2 minutes. Whisk in the chocolate hazelnut spread until well combined and remove from heat.

In a large bowl, whisk together the mascarpone cheese and chocolate hazelnut mixture. Beat together cream, cocoa powder, and sugar in another large bowl using an electric mixer at low speed until just combined. Increase the speed to high and beat until the cream forms soft peaks. Whisk one third of the whipped cream into the mascarpone mixture to lighten it, then fold in the remaining whipped cream until well combined. Spoon the filling onto the cooled shortbread base, gently smoothing the top, then chill, covered, for at least 3 hours.

Make the ganache and glaze cake: Bring cream to a simmer in a small heavy saucepan and remove from heat. Add chocolate and let it stand for 1 minute, then gently whisk until completely melted and smooth. Transfer the ganache to a small bowl and cool, stirring occasionally, until slightly thickened but still pourable, about 20 minutes.

Run a warm thin knife around the inside of your springform pan to release it from the sides. Slide the cake off the bottom of the pan and transfer it to a serving plate. Pour the ganache onto the top of the cake and spread, allowing excess ganache to drip down the sides.