It’s always useful to be able to chuck a sponge cake together. Purists may argue that an airy cake is best made in stages, starting with creaming the butter and sugar, but an extra teaspoon of baking powder makes up for any shortfall in lightness.
Make sure your butter is really soft or you won’t be able to beat the mixture without getting lumps. And line your cake tins before you start making the cake – if the mix has to wait while you find tins, baking paper, etc, it may lose some of its lightness.
We prefer to use ordinary dark chocolate, which has a little added sweetness, for icing our sponge cake – you can use 70% chocolate, but you may need to sweeten the icing with a little sugar if it tastes very strong and bitter.