Mix the flour with the baking powder. Briefly heat the butter in the microwave so that it is slightly runny and add to the flour. Add the remaining ingredients to the butter-flour mixture and stir with the mixer until a liquid dough is formed.
Generously grease a rectangular cake pan. Pour the batter into the mold. Bake the cake at 180 degrees for about 35 minutes. Chopstick sample! Let the cake cool down and then carefully lift it out of the pan. Cover with chocolate if you like.