Pre-heat oven to 180ºC/350ºF/Gas Mark 4. Grease and line two 20cm (8 inch) sandwich tins.Cream together the butter and sugar until light and fluffy. Gradually beat in the eggs and if the mixture starts to curdle, add a little flour.Fold in the remaining flour and the cocoa powder with a metal spoon.Divide the mixture between the two tins and bake for approx. 25 minutes until golden brown and firm to the touch. Remove from oven and leave to cool before turning out onto a cooling rack.To make the butter icing, place the butter in a bowl and beat until soft. Gradually sift and beat in the icing sugar and then add enough milk / water to make the icing fluffy and spreadable. Sift and beat the cocoa powder in to the icing and use it to sandwich the two sponges together.Dust the top of the cake lightly with the sieved icing sugar.