Preheat oven to 175degC.
Melt the butter in a bowl which will hold the whole mixture. Add the eggs, sugar, and milk to the melted butter.
Sift in the carefully measured flour, baking powder and cocoa.
Using an electric beater, beat until the mixture is well mixed, looks creamy and is slightly paler in colour, about 30-60 seconds.
Turn into a 20cm square or round cake tin, or a ring tin, lined with baking paper or a Teflon non-stick spray. (Butter or spray uncovered sides.)
Bake for about 20 minutes or until the centre springs back when pressed.
Dust with icing sugar or ice when cold.