Preheat the oven to 180 °C. Whisk butter, sugar and vanilla until creamy. Add eggs, milk and lemon zest and keep whisking until combined.
Sieve flour and baking powder into the batter. Divide the batter in two halves and put them in separate bowls. Add cocoa powder, grated chocolate and rum to one bowl.
Grease your bundt pan and put some flour into it. Pour half of the light (vanilla) batter into the bundt pan, pour the dark batter on top of it, then add the rest of the light one. Take a fork and swirl it around the batter to create the marble effect. Bake at 180 °C for 50 minutes.
Tipps
This marble bundt cake is perfect with some whipped cream, a hot chocolate, coffee or tea!