Add the butter and sugar into a large mixing bowl, beating with a wooden spoon until mixed together and smooth.
Crack the eggs and whisk them in a mug using a fork. Add to the mixing bowl slowly, stirring as you go. Add a little flour if the mixture gets too runny.
Add the flour and blend in using a metal spoon until everything is mixed in.
Transfer half the mixture into the cake tin and add the cocoa powder to the other half. Mix in and then spoon the chocolate half into the tin. Swirl your spoon on the top of the mixture to create a more marbled effect.
Transfer to the oven for 20-25 minutes or until a knife inserted into the middle comes out clean.
Leave to cool slightly and transfer to a wire rack to cool completely.
It is nice if served with chocolate icing or custard. For Easter, if you used a ring tin you can add mini eggs into the central hole to serve to make the kids even more excited.