Preheat your oven to 325 degrees Fahrenheit. This ensures that your muffins start baking at the right temperature, leading to a perfect rise and golden color.
Peel and mash the bananas in a large mixing bowl. Aim for a smooth consistency, with just a few small lumps for texture. The bananas should yield about 1 to 1.5 cups when mashed.
Add the egg, butter, peanut butter, brown sugar, and vanilla to the mashed bananas. Use a whisk or a fork to mix everything together until it's well combined. The mixture should be creamy and slightly fluffy.
In a separate bowl, combine 1 cup of flour, baking soda, baking powder, and salt. Mix these dry ingredients thoroughly. This helps distribute the leavening agents evenly throughout the batter.
Add the dry ingredients to the wet mixture, stirring gently until just combined. You want to avoid overmixing here, as it can lead to dense muffins.
In a small bowl, take the remaining 1/4 cup of flour and coat the chocolate chips and Reese's peanut butter chips with it. This prevents the chips from sinking to the bottom of the muffins.
Fold the coated chips into the muffin batter gently. Again, mix just until combined to keep the batter light and airy.
Lightly grease a muffin tin or line it with baking cups. This makes for easy removal of the muffins once they’re baked and helps keep the shape intact.
Divide the batter evenly among the 12 cups of the muffin tin. You can fill each cup about 3/4 full to allow room for the muffins to rise.
Bake the muffins for 20 to 25 minutes, or until they turn golden brown and a toothpick inserted in the center comes out clean. Keep an eye on them; every oven is different!
Once baked, let the muffins cool slightly in the tin before transferring them to a wire rack. Enjoy them warm or at room temperature!
