01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or grease lightly.
02 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl until evenly distributed.
03 - Mash bananas in a large bowl. Mix in peanut butter, brown sugar, oil, eggs, and vanilla extract until smooth and well blended.
04 - Stir milk into the banana mixture until fully incorporated.
05 - Add dry ingredients to wet ingredients. Mix gently with a spatula until just combined—do not overmix or muffins will become tough.
06 - 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.
09 - Let muffins cool in pan for 5 minutes, then transfer to a wire rack to cool completely.