Heat the oven to 180C/fan 160C/gas 4. Cream the butter and sugar together, then beat in the eggs and flour. Divide the mix into 2 and add the cocoa to one half and the vanilla to the other. Add a drop of milk to the chocolate mix if it is too stiff.
Spoon into mini muffin tins lined with paper cases, making some chocolate and some vanilla. Bake for 8 minutes. Cool.
Dip the vanilla cakes in dark chocolate and the chocolate ones in white chocolate. Decorate with buttons or sprinkles.