Preheat the oven to 355F (180C) and line a muffin pan with 9 muffin liners.
In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt.
In the bowl of a stand mixer fitted with a paddle attachment (or use a hand mixer and a bowl), cream together the white sugar, unsalted butter, and vanilla extract until light and fluffy. Then, add the oil and peanut butter, and beat until fully incorporated.
Add the egg, and beat until emulsified.
Add the puréed banana and beat to incorporate. Then, add the milk and beat again.
Add the dry ingredient mixture and mix on low just until fully incorporated.
Spread the batter between the muffin liners so they’re ⅔ full and sprinkle chopped peanuts on top.
Bake for 23-25 minutes or until a toothpick inserted in the center comes out clean and the tops are golden.
Remove from the oven and let cool down for 5-10 minutes in the pan. Place onto a cooling rack to cool down completely.