Preheat the oven to 170°C/345°F 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 1 tsp Vanilla Extract followed by the remaining flour and mix again.
Remove half of the cupcake batter to another bowl and add the2 tbsp deZaan Holland Rich Terracotta 20/22% Dutched Cocoa Powder. Stir to combine.
Using two spoons, fill the cupcake liners by adding small spoonsful of 1 tsp Vanilla Extract 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.