Preheat oven to 180 degrees. Brush a pan cake tin with melted butter.
Sift baking powder, flour, baking soda and cocoa into a bowl. Add the butter, water and eggs. Beat with an electric hand mixer on the lowest setting until all ingredients are well mixed together. Then beat on medium speed for another 3 minutes.
Pour the dough into the prepared form and smooth it out. Bake for 45 minutes. Let the cake cool for about 10 minutes and then overturned.
When cooled, cover with powdered sugar or glaze as desired.