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

In a mixing bowl, whisk together the flour, sugar, baking powder, and salt.

In another bowl, combine the melted butter, eggs, vanilla extract, and milk.

Pour the wet ingredients into the dry ingredients and mix until just combined.

Divide the batter in half and mix cocoa powder into one half.

Spoon the batters into the cupcake liners, alternating between vanilla and chocolate.

Use a toothpick to swirl the batters together for a marbled effect.

Bake for 20 minutes or until a toothpick comes out clean.

Let cool before serving.