Step 1

Preheat oven to 350 degrees. Invert the bottom of an 8" springform pan (to make it easier to slide the shortbread base off the bottom) then lock on the sides of the pan. Line the bottom with a round of parchment paper.

Step 2

Place the hazelnuts and granulated sugar in the bowl of a food processor. Pulse until the nuts are finely chopped. Add the flour, butter, cocoa and salt and pulse to form a cohesive dough.

Step 3

Press the dough evenly onto the bottom of the prepared springform pan. Using a fork, prick the base all over and then bake just until it is just dry to the touch, about 18-20 minutes. Cool completely on a wire rack. Remove the sides of the pan and carefully remove the parchment from under the shortbread. Reattach the sides of the pan around the shortbread base. Set aside.

Step 4

Place the water in a small, heavy-bottom saucepan then sprinkle with the gelatin. Set aside until the gelatin is softened, about 5 minutes. Heat the softened gelatin over low heat and stir just until melted. Add the chocolate hazelnut spread and espresso powder and whisk until combined. Remove the pan from the heat source and transfer the mixture to a large mixing bowl. Add the mascarpone cheese and vanilla to the hazelnut mixture and whisk until combined.

Step 5

In the bowl of a stand mixer fitted with the whisk attachment, beat together  1 1/2 cups of heavy whipping cream, 2 tablespoons cocoa powder and 3 tablespoons sugar on low speed just until combined. Increase speed to high and beat until the cream holds soft peaks. Whisk one third of the whipped cream into the mascarpone mixture. Using a rubber spatula, fold in the remaining whipped cream until blended. Spoon the filling into the springform pan on top of the shortbread crust. Using an offset spatula, smooth the top, cover and refrigerate at least 3 hours.

Step 6

In a small saucepan, bring the heavy cream to a simmer over medium heat.  Remove from the heat source and add the chopped bittersweet chocolate.  Allow the mixture to set for 2-3 minutes, then lightly whisk until the chocolate is completely melted.  Set aside to cool until cooled, but still pourable and slightly thickened.

Step 7

To assemble, run a thin warm knife around the inside of the springform pan.  Remove the sides and carefully slide the cake onto a serving plate or cake stand.  Pour the ganache onto the top of the cake and gently spread it to the edges, allowing a little ganache to drip decoratively down the sides.  Garnish with chopped hazelnuts if desired.