To begin, preheat your oven to 350 and spray a muffin tin with non-stick spray.
To a large mixing bowl, add your butter and sugar and beat together with a mixer till creamy. Add your peanut butter, banana, milk, and vanilla and mix together again till everything is well incorporated. Add your eggs and briefly blend them in.
To the same bowl, add your flour, baking powder, baking soda, and salt. Add your cocoa powder to the same bowl through a fine mesh strainer. Stir everything together till you have a smooth, consistent muffin batter.
Fill each muffin tin about 2/3 way full with muffin batter.
To add the peanut butter swirl: Prior to baking, add an overflowing 1/2 teaspoon of peanut butter to the top of each muffin. Use a toothpick and gently swirl the peanut butter throughout the top of each muffin top.
Bake in the oven for 18-20 minutes, or till a toothpick inserted comes out clean. Let cool, then devour!
*I recommend using store bought peanut butter, like Jif or Skippy. Other peanut butters that are more oily may provide different results.
**Stored in an airtight container in the fridge these muffins last about a week.