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 evenly distributed.
03 - In a large bowl, mash bananas thoroughly. Mix in peanut butter, brown sugar, oil, eggs, and vanilla extract until smooth and well blended.
04 - Stir milk into the wet mixture until fully incorporated.
05 - Add dry ingredients to wet ingredients. Mix gently with a spatula until just combined—do not over-mix or muffins will become dense.
06 - Gently fold in chocolate chips or chopped peanuts if desired.
07 - Divide batter evenly among muffin cups, filling each about 3/4 full.
08 - Bake for 18-22 minutes, or until a toothpick inserted in the center of a muffin comes out clean.
09 - Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.