Stir butter and sugar until soft. Add egg, flower mixed with baking powder and milk. Pour the batter in a large oven tray covered with grease proof paper. Bake for 15 minutes in 200º. Let cool.
Melt the butter, add icing sugar and cacao. Mix and and water. Whisk the egg whites, melt the chocolate and add to the sugar mix. Stir until smooth and spread over the cake.
Optional: – add chopped almond to the chocolate spread – sprinkle with different kinds of chocolate sprinkles