Preheat your oven to 350°F (175°C). If using muffin liners, place them in the muffin tin; otherwise, grease the tin lightly with cooking spray.
In a large mixing bowl, combine the mashed bananas, peanut butter, granulated sugar, and brown sugar. Mix well until smooth.
Add the eggs and vanilla extract to the banana mixture, and whisk until combined.
In a separate bowl, whisk together the flour, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture, alternating with the milk. Mix until just combined; do not overmix.
Divide the batter evenly among the muffin cups, filling each about ¾ full.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Remove the muffins from the oven and allow them to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.