Preheat the oven to 425°F (220°C). Line a 12-cup muffin tin with muffin liners.
2
In a large mixing bowl, mash the ripe bananas using a fork until smooth.
3
Add vegetable oil, beaten egg, granulated sugar, smooth peanut butter, and milk to the mashed bananas. Stir until well combined.
4
Add the store-bought muffin mix to the wet ingredients. Gently fold in the dry mix until just combined. Avoid over mixing to ensure fluffy muffins.
5
Scoop the batter evenly into the prepared muffin tin, filling each liner 2/3 full.
6
Bake in the preheated oven for 5 minutes, then lower the heat to 350°F (180°C) without opening the oven door. Continue to bake for an additional 15-18 minutes, or until a toothpick inserted in the center of a muffin comes out clean.
7
Remove muffins from the oven and allow them to cool in the tin for a few minutes before transferring them to a wire rack to cool completely.