Preheat your oven to 350°F (180°C). Grease a 9-inch (22cm) cake pan and line the bottom with parchment paper.
In a large bowl, whisk together the flour, baking powder, baking soda, salt, and ground walnuts.
In another bowl, whisk the eggs with the granulated sugar until light and fluffy. Then, add the melted butter, milk, and vanilla sugar.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Pour the batter into the prepared cake pan and bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.
Let the cake cool completely in the pan before slicing it in half horizontally.
Prepare the walnut cream: whip the heavy cream until stiff peaks form. Gently fold in the Nutella and powdered sugar.
Spread the Nutella walnut cream evenly over the bottom cake layer, then top with the second layer.
Make the chocolate ganache: heat the heavy cream until just simmering. Pour it over the finely chopped dark chocolate. Let it sit for a minute, then stir until smooth and glossy.
Pour the chocolate ganache over the cake, coating it evenly. Sprinkle with chopped walnuts. Refrigerate for at least 1 hour to allow the ganache to set.
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