Preheat the oven to 325°F (165°C). Grease a 9-inch springform pan and line the bottom with parchment paper.
In a large bowl, whisk together the hazelnut flour, almond flour, cocoa powder, granulated sweetener, baking powder and salt, breaking up any lumps of cocoa.
Add the melted butter, eggs, sour cream, almond milk and vanilla extract. Mix until the batter is smooth and glossy, then spread it evenly in the springform pan.
Bake for 28-32 minutes, until the center is set and a toothpick comes out with just a few moist crumbs. Cool in the pan for 20 minutes, then release the sides and cool completely.
While the cake cools, toast the chopped hazelnuts in a dry skillet over medium heat for 3-4 minutes, stirring often, until fragrant and golden. Set aside to cool.
For the hazelnut cream, add the cold mascarpone, cold heavy cream, 1/3 cup chocolate hazelnut spread, powdered sweetener and cocoa powder to a large bowl. Beat on low for 30 seconds, then on medium-high for 2-3 minutes until thick, stiff peaks form.
Pile the cream on top of the cooled cake and swirl it out toward the edges. Warm the remaining 2 tbsp hazelnut spread for a few seconds until pourable and drizzle it over the cream. Scatter the toasted hazelnuts on top and chill for 1 hour before serving.