Grease two 8-inch / 20 cm square cake tins with oil and line the bottoms with parchment paper / baking paper.
Sift flour and baking powder together in a medium bowl and set aside.
Place softened butter in a large bowl and add sugar. Cream together using an electric hand whisk till light and fluffy. It is also possible to use a stand mixer.
Add the eggs one by one, beating for 30 - 45 seconds after each addition.
Add the vanilla essence and stir through.
Add the flour in three separate parts and fold it through using a spatula.
One part of the batter (⅓rd) will be used to make the plain cake and two parts (⅔rd) of the batter will be used to make the chocolate cake.
Divide the mixture into two bowls – one part (~250g) in one bowl for the plain cake and two parts (~500g) in the other bowl for the chocolate batter.
Stir cocoa powder and milk together to form a smooth paste, and then fold it through the larger portion of batter to make a chocolate batter.
Pour the plain batter in one square tin, and the chocolate batter in the other square tin.
Bake the cakes for 20 – 25 mins until a skewer inserted into the centre comes out clean. Let the cakes cool in the pans for 30 minutes, then invert the cakes onto a rack to cool. Peel off the parchment.