Preheat the Oven: Start by preheating your oven to 350°F (175°C). Grease a round cake pan or line it with parchment paper.
Melt the Chocolate: In a heatproof bowl, melt the dark chocolate over a pot of simmering water, stirring until smooth. Set it aside to cool slightly.
Beat the Eggs and Sugar: In a large mixing bowl, beat the eggs and sugar together until light and fluffy. This should take about 4-5 minutes.
Combine Ingredients: Gently mix the melted chocolate and soft butter into the egg and sugar mixture until fully incorporated.
Add Dry Ingredients: Carefully fold in the ground hazelnuts, baking powder, vanilla sugar, and a pinch of salt. Ensure the mixture is combined without overmixing.
Pour into Pan: Pour the batter into the greased cake pan, smoothing the top with a spatula.
Bake: Place the pan in the preheated oven and bake for about 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Cool and Serve: Once baked, let the cake cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely. Decorate with your choice of toppings and enjoy!