Cream butter and sugar until fluffy.
Beat eggs and add gradually.
Add warm water and beat well.
Add sifted flour and baking powder, and stir thoroughly until the mixture will spread easily.
Put into two well greased and floured sandwich tins. (Don't forget to put a small piece of greaseproof paper at the bottom to ease turning out.)
Bake in a moderate oven for about 20 minutes.