Beat butter or margarine and sugar till light and fluffy. Add 1 egg at a time beating in between to form peaks. Add vanilla essence. Sieve flour and baking powder in a seperate bowl. Add three tablespoon of flour mixture to the butter at a time while continuing to mix till you are done with all the flour. Scoop half cup of batter put in a bowl, add cocoa and hot water mix well. Then pour half of vanilla batter in baking tray then chocolate batter use cocktail stick to spread the chocolate batter, pour the remaining vanilla batter on it. Bake in a pre heated oven at 170 C till done.