Preheat oven to 350°F (180°C). Grease a 9-inch (22 cm) cake pan and line the bottom with parchment paper.
In a large bowl, whisk together the flour, baking powder, baking soda, cinnamon, salt, and ground walnuts.
In another bowl, beat the eggs with the granulated sugar and brown sugar until light and fluffy. Add the oil and vanilla sugar and mix well.
Mash the ripe bananas and stir them into the egg mixture.
Gradually add the dry ingredients to the banana mixture, mixing until just combined.
Pour the batter into the prepared cake pan and bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean.
While the cake is baking, prepare the cream: whip the heavy cream until stiff peaks form, then gently fold in the mascarpone and powdered sugar.
Let the cake cool completely, then evenly spread the mascarpone cream over the top.
Sprinkle with chopped walnuts and thin banana slices, then refrigerate for another hour.
Slice and serve, and enjoy the soft, walnut-rich banana cake!
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