Preheat oven to 350°F (180°C). Grease and dust a 9-inch (22 cm) cake pan with cocoa powder.
Separate the eggs. Beat the egg whites with a pinch of salt until stiff peaks form.
In a separate bowl, cream the egg yolks with the granulated sugar and vanilla sugar until pale and fluffy. Add the melted butter and milk and mix well.
In another bowl, whisk together the flour, cocoa powder, baking powder, and ground walnuts.
Gradually add the dry ingredients to the egg yolk mixture, mixing until just combined. Gently fold in the beaten egg whites, being careful not to deflate the batter.
Pour the batter into the prepared cake pan and spread evenly with a spatula.
Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.
Remove from oven and let cool in the pan for 10 minutes before inverting onto a wire rack to cool completely.
Before serving, dust with powdered sugar or drizzle with chocolate sauce.
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