In a mixing bowl combine the flour, cocoa powder, granulated sugar, baking powder, baking soda, and salt. Stir together.
In a larger bowl combine the eggs, buttermilk, vegetable oil, and vanilla extract. Stir to combine.
Gradually mix the dry ingredients into the wet ingredients, mixing just until combined. The batter will be thick.
Cover the bowl with a towel and allow the batter to rest for 15 minutes.
Preheat oven to 425℉
Prepare a 12-count muffin pan with liners. Pour 2 tbsp of batter into each muffin cup.
Heat the peanut butter in the microwave until melty. Place ½ tablespoon of peanut butter into each muffin cup and use a toothpick to swirl. Add an additional 4-6 tbsp of batter to each.
Top each muffin with ½ tbsp of peanut butter, and swirl with a toothpick.
Bake for 7 minutes at 425℉. Reduce the temperature to 350℉ and bake for an additional 12-14 minutes, or until a toothpick inserted into the center comes out clean.