Preheat your oven to 350°F (175°C) and line a muffin tin with cupcake liners.

In a medium bowl, whisk together the all-purpose flour, baking powder, and salt until well combined.

In a large mixing bowl, cream the softened unsalted butter and granulated sugar together using an electric mixer for about 3-5 minutes until light and fluffy.

Add the eggs one at a time to the creamed mixture, mixing well after each addition, then add the vanilla extract.

Gradually add the dry ingredient mixture and the milk to the wet ingredients, alternating between the two.

In a small bowl, mix the cocoa powder with hot milk or hot water, then fold in half of the vanilla batter to create the chocolate batter.

Spoon the vanilla batter into the lined muffin tin halfway, add a spoonful of chocolate batter on top, and swirl gently with a toothpick.

Bake for 18-22 minutes until a toothpick inserted comes out clean, then cool in the tin for 5 minutes before transferring to a wire rack.