We went with a Star Wars theme since with white and milk chocolate together we have the light side and the dark side 
The recipe is for a milk and white chocolate zebra cake:
4 large free-range eggs
250g/9oz golden caster sugar
100ml/3½fl oz milk
250ml/9fl oz sunflower oil
1 tsp vanilla extract
475g/1lb 1oz self-raising flour
1 tsp baking powder
25g/1oz cocoa powder
1 115g bag of Cadbury's mixed buttons
Preheat the oven to 180C/350F/Gas 4. Grease a 23cm/9in cake tin.
Put the eggs and sugar into a large bowl with the milk, oil, vanilla extract and 175g/6oz of the flour. Whisk until smooth.
Separate the Cadbury's buttons into milk and white, and crumble them up a bit.
Pour half of the mixture into a separate bowl and stir in ½ tsp baking powder and 175g/6oz flour and add the white half of your crushed Cadbury's buttons. In the other bowl, mix the cocoa, 125g/5oz flour and ½ tsp baking powder with the milk buttons.
Spoon two tablespoons of the white mixture into the centre of the cake tin. Straight away spoon two tablespoons of the chocolate mixture on top. Continue doing this until all the mixture has been used – you will end up with lots of alternate rings of mixture in the tin.
Cook for 40-50 minutes, until the cake is cooked through – a skewer inserted into the centre should come out clean.
For the chocolate buttercream:
85g butter
175g icing sugar
2 tablespoons cocoa powder
1 tablespoon milk
Mix butter and icing sugar well until smooth.
Add in cocoa powder and milk and mix again until smooth.
Spread over cake
Our cake was not quite as stripy as it should have been, as over-enthusiastic DS wanted to give the mixture a "good mix" after it had been carefully layered! 
We then decorated with the other bag of buttons. If you squint a bit, you'll see that is, in fact, a stormtrooper 