Preheat oven to 180°C. Beat butter until fluffy. Slowly add sugar, vanilla extract, eggs and rum.
In a separate bowl combine flour, salt and baking powder. Sift into egg mixture. Add as much milk as necessary until you like the consistency. Add 2/3 of dough into greased baking pan.
Add cocoa powder to remaining dough. Mix and add milk if necessary. Add this dough to the baking pan and gentle mix with a fork to receive marble pattern. Bake for 50 to 60 minutes at 180°C or until a wooden stick comes out clean.