Preheat the oven to 165 C. Lightly grease the pan with some butter.

Mix the flour and the baking powder together and sieve. Add in salt and mix well. Set aside.

Use an electronic beater to mix butter and sugar until well combined or pale yellow in color. Add in the first egg. Beat well after each addition of egg until creamy. Scrape down the sides and add in vanilla essence and fold in the flour into the mixture and mix well. Finally, add in the milk.

Divide the batter into two portions. Stir the cocoa powder into one portion and mix well. Transfer the plain batter into the greased baking pan. Shake it lightly to distribute evenly. Pour the cocoa batter in the middle of baking pan, and lightly swirl the cocoa batter with a butter knife.

Bake until golden brown and cooked, about 30 minutes.