Preheat oven to 350°F (180°C) and line a cake pan with parchment paper.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time.
In a separate bowl, combine the ground walnuts, flour, and baking powder. Gradually add this mixture to the butter-egg mixture, mixing until just combined.
Pour the batter into the prepared cake pan and bake for 35-40 minutes, or until a wooden skewer inserted into the center comes out clean.
Let the cake cool completely before removing it from the pan. Meanwhile, prepare the whipped cream with the vanilla sugar.
Frost the top of the cake with whipped cream and sprinkle generously with fresh blueberries. Refrigerate for at least 1 hour before serving.
Slice the cake and garnish with extra walnuts or blueberries, if desired.
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