Preheat the oven to 350°F (175°C). In a food processor, pulse the hazelnuts until finely ground. Add butter, sugar, flour, cocoa powder, and salt, and blend until combined. Press the mixture into the bottom of a greased 9-inch springform pan. Bake for 10-12 minutes, then let cool.

Chocolate Mousse Preparation

Prepare the Mousse

Melt dark chocolate over a double boiler or microwave and allow to cool slightly. In a separate bowl, whip heavy cream with sugar and vanilla until soft peaks form. In another bowl, whisk egg yolks until thick. Soften gelatin in water, then stir into the warm chocolate. Fold the chocolate mixture into the egg yolks, then gently fold in the whipped cream until fully combined. Pour the mousse over the prepared crust and refrigerate for at least 4 hours to set.

Raspberry Puree Preparation

Make the Raspberry Puree

In a blender, combine fresh raspberries, sugar, and lemon juice. Blend until smooth, then strain through a fine mesh sieve to remove seeds. Set aside.

Chocolate Sauce Preparation

Prepare the Chocolate Sauce

In a small saucepan, heat heavy cream until just simmering. Remove from heat and add dark chocolate and butter, stirring until smooth and glossy. Allow to cool slightly.

Assembly

Assemble the Cake

Once the mousse has set, remove the cake from the springform pan. Serve slices drizzled with raspberry puree and chocolate sauce.