Melt the butter in the warm milk and add the baking powder. Add a little at a time the flour, the sugar and lastly the cocoa. Mix all the ingredients until the batter is quite thick. Pour the mixture into a greased and floured cake pan and bake in a preheated oven at 180°C (355°F) for about 40 minutes. Remove from the oven and, when it has cooled, sprinkle with powdered sugar.