First, mix all the dry ingredients, except the cocoa, in a large bowl.
Next, add the wet ingredients to the dry mixture and mix well until you have a smooth batter.
Divide the batter into two bowls and add the cocoa to one of them, mixing until well incorporated.
Grease a cake tin with butter and pour the cocoa batter into the base.
Then, pour the white batter on top of the cocoa batter.
Use a spatula or a toothpick to create a marbled effect in the batter by gently swirling the two colours.
Bake the cake in a preheated oven at 170°C for about 40-50 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool before removing it from the tin and serving.