• Preheat oven to 325 F. Grease and line a 9-inch springform pan with parchment paper.
• In the bowl of a food processor, grind the chocolate and hazelnuts together until a paste forms.
• Add the butter and sugar to the ground hazelnuts and chocolate mixture and pulse until thoroughly incorporated.
• Add the grand marnier, then the eggs, one by one.
• Scrape down the bowl, then add the flour to the wet ingredients.
• Use a rubber spatula to scrape the cake batter into the prepared pan. Use an offset spatula to level the batter as it will set in the shape that you put it in the oven!
• Bake 20 to 25 minutes until a cake tester inserted into the center of the cake comes out clean.
• Turn onto a wire rack, and let fully cool.
• Dust the top of the cake with powdered sugar, or top with a large dollop of whipped cream.

For the whipped cream

• In the bowl of a stand mixer, using the whisk attachment, beat the cream, icing sugar, and vanilla extract on medium speed until soft peaks form. Use a rubber spatula to plop on the cake, or put the whipped cream in a piping bag fitted with a star tip for a more decorative effect.

To decorate:

• We feel that this cake is so rich that it doesn't require a chocolate frosting, but enjoy with a side of whipped cream, chocolate whipped cream, or just sprinkled with powdered sugar.