Preheat the oven to 425°F. Line a muffin tin with liners,
Mash your bananas in a large bowl.
Add in the oil, milk, egg, and vanilla to the bowl. Beat well.
Add in the sugar and peanut butter, beat well until smooth
Stir together the four, salt, and baking soda in a separate smaller bowl.
Pour the flour mixture into the banana mixture and stir until just combined, there will be lumps, and that is okay.
Divide the batter evenly in the muffin tin, and bake for 5 minutes. Turn down the heat to 350°F without opening the oven door, and continue to bake for 12 - 15 minutes, or until a toothpick inserted in the middle of a muffin comes out clean.
Video
Notes
Use creamy peanut butter. Natural peanut butter can work, as long as you stir the peanut butter well first.
Store in an airtight container for up to 3 days at room temperature or 3 months in the freezer.
Use ripe bananas that are at least speckled with brown spots. The more brown spots, the better.