Step 1: Prepare the Vanilla Batter

1. Preheat your oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners.
2. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
3. In a separate bowl, combine the sugar, buttermilk, melted butter, egg, and vanilla extract. Mix until smooth.
4. Gradually fold the dry ingredients into the wet ingredients until just combined. Avoid overmixing.

Step 2: Prepare the Chocolate Batter

1. In another medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt.
2. In a separate bowl, mix the sugar, buttermilk, melted butter, egg, and vanilla extract until smooth.
3. Fold the dry ingredients into the wet ingredients until just combined.

Step 3: Swirl the Batters

1. Using a spoon or cookie scoop, alternate adding dollops of vanilla and chocolate batter into each muffin cup. Fill each cup about three-quarters full.
2. Use a toothpick or skewer to gently swirl the batters together, creating a marbled effect.

Step 4: Bake the Muffins

1. Bake in the preheated oven for 18–20 minutes, or until a toothpick inserted in the center comes out clean.
2. Let the muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.