Preheat oven to 400°F. Arrange muffin liners in a muffin pan.
In a medium bowl, mash together banana and room temperature egg with a fork until fairly smooth. Whisk in melted butter. Set aside.
Whisk dry ingredients (flour, brown sugar, cocoa powder, baking powder, salt) together in a small bowl.
Fold half of the dry ingredients into the wet ingredients. Then add the greek yogurt and continue folding until almost combined.
Add the remaining dry ingredients and fold gently until combined and no dry patches or streaks of yogurt remain. Cover and let rest 30-60 minutes.
Use a .75 oz (1½ tablespoon) cookie scoop to fill each lined muffin tin about ⅓ of the way full. Use a toothpick to spread the muffin batter out so it covers the bottom of each tin.
Dollop creamy peanut butter into the center of each muffin using a .25 oz (1½) teaspoon scoop.
Divide the remaining muffin batter between the muffin tins, covering the peanut butter.Use a toothpick to give each muffin a quick swirl to hide the peanut butter in the center. It's okay if some peanut butter is visible on top, you just want it mostly under the top layer of muffin batter.
Top muffins generously with mini chocolate chips. Use the back of a spoon to press the chocolate chips into the batter.
Bake 22-25 minutes at 400°F until muffins are no longer jiggly in the center. An instant read thermometer should read 190°-200°F.
Let cool briefly in muffin tins, then transfer to wire rack to finish cooling.