Preheat oven to 180°C and grease the baking pan. Cream butter and sugar; add eggs gradually till the mixture becomes light and fluffy. Add vanilla extract and mix. In separate bowl sift together flour, baking powder and salt. Add the flour mix to egg batter.

Heat water and add it to cocoa powder, make a smooth paste. In a separate bowl take 1/3 of the batter and mix the cocoa mixture and stir it well.

In the pan add both the batters simultaneously and in the end swirl the mixture with skewer if you want the swirl marble effect (I left it like that). Bake for 35-40 minutes/ or till the knife comes out clean