Preheat the oven to 170C/345F and line 2 12-hole cupcake trays with 16 small cupcake liners.
In the bowl of an electric mixer, add the butter and sugar and mix on a medium speed for 5 minutes until the butter is light and creamy.
Sift the flour and baking powder together and set aside. Turn the mixer down to a low speed and crack the eggs in one at a time.
Once the eggs are fully incorporated add half of the flour and mix to combine, add the milk and vanilla extract followed by the remaining flour and mix again.
Remove half of the cupcake batter to another bowl and add the cocoa powder. Stir to combine.
Using two spoons, fill the cupcake liners by adding small spoonsful of vanilla and small spoonsful of chocolate batter to each liner until 2/3 full.
Use a cake skewer or a small knife to marble the two batters together and bake for 20 minutes or golden brown and springy to the touch.
Allow the cupcakes to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.