This impressive rainbow cake ... put the 20cm/8in cake board on top. Put a dab of buttercream on the cake board and place the purple sponge on top. Spread a ¾cm/½in thick layer of buttercream ...

When you make a cake (like this rainbow cake), cupcakes ... or maybe you had to level off layers of cake to make each layer the same size, and you're left with a pile of perfectly good cake ...