Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with cupcake liners.
In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
In a large bowl, beat the butter and sugar together until light and fluffy. Add the eggs one at a time, mixing well after each. Stir in the vanilla.
Gradually add the dry ingredients to the wet ingredients, alternating with the milk, starting and ending with the flour mixture. Mix until just combined.
Separate one-third of the batter into another bowl and mix in the cocoa powder to create the chocolate batter.
Fill each cupcake liner with a mixture of the vanilla and chocolate batters to create a marbling effect, using a toothpick to swirl gently.
Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool before serving.
Notes
For best results, ensure ingredients are at room temperature.