Beat butter and sugar till white.
Beat eggs, then beat with butter and sugar mixture.
Add milk and vanilla essence.
Sift the flour and baking powder and mix in quickly and lightly, not beating.
Divide mixture into three parts. Leave one plain. Mix cocoa into one. And colour the other part.
Cook each part separately in a hot oven for 12 minutes. I've used a ring tin. If using a round tin, you may need to cook a little longer.
When cold, join parts with jam, ice and decorate.