Preheat your oven to 350℉. Invert the bottom of an 8-inch springform pan (to make it easier to slide the shortbread base off later) and lock on the sides. Line the bottom with a round of parchment paper.

In a food processor, pulse the hazelnuts and granulated sugar until the nuts are finely chopped. Add the flour, butter, cocoa, and salt, then pulse until a cohesive dough forms.

Press the dough evenly onto the bottom of the prepared springform pan. Prick the base all over with a fork and bake until 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.

While the crust is baking and cooling, prepare the mousse:

Place the water in a small saucepan and sprinkle with the gelatin. Let it sit until softened, about 5 minutes. Heat the softened gelatin over low heat and stir until melted. Add the chocolate hazelnut spread and espresso powder, whisking until combined. Remove from heat and transfer to a large mixing bowl. Add the mascarpone cheese and vanilla extract, whisking until smooth.

In the bowl of a stand mixer with the whisk attachment, beat together 1 1/2 cups heavy whipping cream, 2 tablespoons cocoa powder, and 3 tablespoons sugar on low speed 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. Fold in the remaining whipped cream with a rubber spatula until blended. Spoon the filling into the springform pan on top of the shortbread crust. Smooth the top with an offset spatula, cover, and refrigerate for at least 3 hours.

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

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 over the top of the cake, spreading it to the edges and allowing a little to drip down the sides decoratively. Garnish with chopped hazelnuts if desired.