Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners.
In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking soda, and salt until well combined.
In another bowl, mix the buttermilk, vegetable oil, egg, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
Gently fold in the chocolate chips.
In a small microwave-safe bowl, warm the peanut butter for about 20-30 seconds until it’s easier to swirl.
Fill each muffin cup about two-thirds full with the batter, then drop a teaspoon of peanut butter on top of each muffin.
Use a toothpick or a knife to swirl the peanut butter into the batter.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.