Preheat your oven to 180°C (160°C fan) and line a 12-hole muffin tray with large cupcake or muffin cases.
In a bowl, cream together the butter and caster sugar with an electric mixer until nice and smooth. Add in the eggs and flour, then beat again on medium speed for about 20–30 seconds until everything is combined.
Split the batter into two bowls. Add the vanilla to one half and the cocoa powder to the other, then beat each bowl until smooth. The batter might be a little thick, so loosen it with about 2 tablespoons of whole milk in each bowl.
Now for the fun part — creating that gorgeous marble effect! Spoon small amounts of each batter randomly into the cases, filling them about ⅔ full. Pop them in the oven and bake for 18–20 minutes, or until cooked through. Once done, transfer the cupcakes to a wire rack to cool.
Beat the butter for a couple of minutes until it’s really smooth and light. Gradually add the icing sugar, half at a time, beating well between each addition. Add in the cocoa powder and keep beating until your frosting is fluffy and luscious — about 3–4 minutes.
Once ready, pipe your buttercream onto the cupcakes using your favourite tip — I love a large star!
Finally, get creative! Decorate with sprinkles, a chocolate drizzle, or however you like. Enjoy your beautiful, delicious cupcakes!
