Step 1

Preheat oven to 350°. Lightly butter the springform pan. Pulse graham crackers, hazelnuts and brown sugar in processor until finely ground. Add butter and Frangelico and continue processing until crumbs are evenly moistened. Press crumb mixture onto bottom and partially up the sides of prepared pan. Bake 10 minutes. Cool on rack.

Step 2

Sprinkle gelatin over Frangelico in a small heavy saucepan and let stand until softened, about 5 minutes. Heat gelatin mixture over low heat, stirring, just until gelatin is melted, about 2 minutes. Whisk in chocolate hazelnut spread until spread is softened and thoroughly combined with Frangelico/gelatin. Remove from heat.

Step 3

Whisk together mascarpone and chocolate hazelnut mixture in a large bowl. Beat together cream, cocoa powder, and sugar in another large bowl with an electric mixer or in a stand mixer at low speed until just combined, then increase speed to high and beat until cream holds soft peaks. Whisk one third of whipped cream into Nutella mixture to lighten, then gently fold in remaining whipped cream until well combined. Spoon filling onto hazelnut base in pan, gently smoothing the top, then chill, covered, at least 3 hours.

Step 4

Towards the end of the refrigeration period, prepare the ganache. Heat the cream in the microwave until very hot. Add the chocolate and let site for several minutes. When the chocolate melts, stir to combine thoroughly. Set aside to cool a bit.

Step 5

When the mousse is set, run a warm, thin knife around the edge of the springform pan to loosen, and then remove the sides. Place the cake (still on the bason a piece of parchment paper. Either coat the top of the cake with the ganache, or place ganache into a plastic baggie with one corner snipped off to create a small hole. Pipe the ganache over the cake in a design of your choice.

Step 6

Allow the ganache to set. Use an offset spatula to gently loosen and remove the cake from the springform pan base. Serve immediately, or refrigerate, covered.