1. Preheat the oven to 180°C (350°F). Thoroughly grease a large bundt cake tin* with butter to ensure the cake releases easily later. Separate the eggs and beat the egg whites until stiff peaks form. Mash the bananas and set them aside.

2. In a large bowl, mix the butter and sugar until light and fluffy. Add the egg yolks and mix until well combined. Stir in the rum (optional) and vanilla extract. Add the mashed bananas and mix in as well.

3. Combine flour, hazelnuts, baking powder, and salt. Add these dry ingredients to the large bowl in two batches, alternating with the milk, and mix only briefly after each addition. The ingredients should be well combined, but be careful not to overmix the batter. Gently fold in the stiff egg whites in two batches. Try to keep as much volume as possible.

4. Transfer about half of the batter to a separate bowl, sift the cocoa powder on top, and gently stir it in. Again, mix only as much as necessary. Spoon a small amount of the dark batter into the prepared pan, then spoon some of the light batter over it and spread evenly. Repeat this process until all the batter has been used. Lightly swirl the two batters together with a knife or similar tool to create a marbled effect.

5. Bake the cake in the preheated oven for about 55–60 minutes. Check with a wooden skewer – the cake is done when it comes out clean.

6. Take the cake out of the oven and let it cool down inside the bundt cake tin for about 15 minutes. Carefully remove the cake from the baking tin and let it cool down completely on a wire rack. Decorate the cake to your liking with some cake glaze and sliced hazelnuts (optional). Store leftovers well-wrapped at room temperature (max. 3–4 days).