In a large mixing bowl, add all ingredients except the cocoa powder.
Mix everything with a hand mixer for about 5 minutes, until you get a smooth batter.
Grease a 26 cm bundt cake pan well and dust with flour.
Pour half of the batter into the pan.
Add cocoa powder to the remaining batter (amount to your liking), stir until well combined.
Pour the chocolate batter over the light batter in the pan.
Use a fork to swirl the two layers together gently, creating a marble effect.
Bake in a preheated oven at 180°C (356°F), top and bottom heat (no fan), for about 60 minutes.

After cooling, dust with powdered sugar or top with chocolate glaze.
This cake freezes wonderfully – so feel free to bake two at once!