Preheat the oven to 350F or 175C
Grease baking pan (9'' pan will do). You can do this with butter or oil
In a bowl, mix the flour, baking powder and salt. Set Aside
In another bowl, add the sugar, egg and oil and mix until creamy/pale yellow
Add vanilla and mix for about 30secs
Add milk and mix
Add flour mixture from the other bowl and mix thoroughly
Reserve 1 cup of batter and pour the remaining in the baking pan
Add cocoa powder in reserved batter and mix thoroughly. If you feel the cocoa batter is too thick, you may add 2 or 3 tablespoons of milk.
Pour the cocoa batter in the pan. Using a knife, create swirls (mixing the coco and vanilla batter)
Bake for about 30 to 50minutes or until an inserted toothpick comes out clean