Preheat oven to 350 F. Line the bottom of a 8-inch round pan with removable bottom with parchment paper. Spray the walls and bottom with cooking spray. Set aside.
If you are making your own hazelnut flour place hazelnuts in a food processor and pulse 7-10 times until finely ground. Be careful not to make nut butter.
On the bowl of a stand mixer, fitted with the whisk attachment beat the egg yolks and sweetener at medium-high speed for 5 minutes, until pale and thick. Carefully and slowly add in the melted chocolate and beat to combine.
Fold in the ground hazelnuts. Set aside.
Beat egg withes until stiff peaks (4-5 minutes at medium-high speed).
Fold the egg whites into the nut mixture.
Pour batter into the pan. Bake for 25-30 minutes, until the top is set and toothpick inserted into the middle comes out clean.
Let the cake cool for 10 minutes, then remove from the pan. Let it cool completely on a wire rack.
At this point you can either garnish with berries and frosting or slice through the middle into two thin cake layers and add frosting in between them.