Beat the softened butter, sugar and eggs until frothy. Gradually stir in the flour, cocoa powder, baking powder and cinnamon. Add the rum - if children prefer to eat milk - and stir well until a smooth dough is formed.
Peel and grate the apples, fold into the batter. Bake in a greased pan at 170 ° C (top / bottom heat, preheated) for 1 hour.