Marmorgugelhupf (Marbled Bundt Cake)

Directions

1. Pre-heat your oven to 180°C (350°F).
2. Prepare a 23 cm (9 inch) bundt cake tin by buttering it and then dusting it with the bread crumbs.
3. Add the baking powder to the flour and whisk to combine.
4. In the bowl of a stand mixer whip the egg whites together with a pinch of salt to stiff peaks. Transfer to a medium bowl and set aside.
5. In the same bowl that was used to whip the egg whites cream butter and sugar until pale and fluffy. On low speed add the egg yolks one by one and then the lemon zest and vanilla.
6. Next add in the flour until it's fully incorporated. Optionally add 3-4 tbsp of milk until the batter is thick yet creamy and not too liquid.
7. Fold the reserved whipped egg whites into the batter.
8. Transfer 1/3 of the batter to the prepared cake tin, 1/3 to the bowl that held the egg whites and leave one 1/3 in the bowl of the stand mixer.
9. In a small bowl combine cocoa powder, milk and rum, if using. The consistency should resemble a thick yet smooth paste, like Nutella.
10. Add the cocoa paste to the batter in the stand mixer bowl and mix on low speed until fully combined.
11. Dollop the cocoa batter into the cake tin before adding the remaining plain batter. Using a dull knife swirl the batters together but be careful not to over-mix them.
12. Bake the cake for 50 minutes, until a toothpick comes out clean.
13. Let the cake cool in the tin for 10 minutes before turning it out and leaving it to cool until it's just slightly warm.
14. Dust with confectioners sugar and serve.