Preheat oven to 180°C. Beat butter frothy with 50g sugar. Separate eggs into egg yolks and egg whites. Sift baking powder and flour. Gradually add the yolks to the butter-sugar mixture, add the flour and as much milk necessary to make a firm dough that only slowly separates from the cooking spoon. Beat the egg white stiff with the remaining sugar and fold into the mass. Grease a Gugelhupf tin with butter and dust with flour. Fill in the dough and bake in a preheated oven for approximately 45 minutes. Turn off the oven, open the door and let the cake cool down for further 10 minutes. Put onto a grid until cool. Put the icing sugar into a sieve and sprinkle over the cake.