Process the hazelnuts and sugar in a food processor until finely chopped. Add the flour, butter, cocoa powder, and salt. Pulse just until a dough forms.

Press the dough evenly into bottom of the prepared pan. Using a fork, poke holes over the surface of the dough. Bake 18-20 minutes, or until dry to the touch (Careful! It will be hot!) Cool completely (about 30 minutes). Remove the sides of the pan and carefully slide the parchment paper from underneath the crust. Reattach the sides.

Place the water in a 1 - 1 & 1/2 quart sauce pan. Sprinkle the gelatin over the water. Let stand about 5 minutes, or until softened.

Heat over low heat, stirring constantly, just until the gelatin has dissolved (about 2 minutes). Whisk in the Nutella until combined. Remove from heat.

In a large bowl, whisk the mascarpone and the Nutella mixture. In a separate bowl, beat the cream, cocoa powder, and sugar with an electric mixer on low speed just until combined. Then, increase mixer speed to high and beat until stiff peaks form.

Whisk 1/3 of the whipped cream into the mascarpone mixture to lighten. Then, gently fold in the remaining whipped cream. Spoon the filling on top of the crust, and spread evenly. Refrigerate at least 3 hours.

Place the cream in a small heavy saucepan and bring to a simmer. Remove from heat. Add the chocolate and let stand for 1 minute.

Then, whisk gently until the chocolate has melted and the mixture is smooth. Transfer to a small bowl to cool. Stir occasionally, until slightly thickened but pourable (about 20 minutes).

Run a warm knife between the cake and pan. Remove the sides of pan. Transfer the cake to a serving plate. Pour the ganache over the cake. Spread over the top of the cake, allowing the excess to run over the sides.