01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or lightly grease with cooking spray.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt until well blended.
03 - In a large bowl, mash bananas thoroughly. Add peanut butter, brown sugar, oil, eggs, and vanilla extract. Mix until smooth and incorporated.
04 - Stir milk into the wet mixture until fully combined.
05 - Add dry ingredients to wet ingredients. Fold gently with a spatula until just combined—do not overmix or muffins will become tough.
06 - Gently fold in chocolate chips or chopped peanuts if desired.
07 - Divide batter evenly among prepared muffin cups, filling each about 3/4 full.
08 - Bake for 18-22 minutes until a toothpick inserted in the center comes out clean or with just a few moist crumbs.
09 - Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.