1. In a large bowl, whip the peanut butter (either by hand with a whisk or with a electric mixer) until it becomes light and fluffy. Then add the the oil and whisk until everything’s combined.
2. Add in the egg, white sugar and brown sugar and mix until the sugar is dissolved.
3. Mash 2-3 ripe bananas (2 if large, 3 if medium sized) and add them into your batter and mix to combined. The more ripe the banana the better.
4. Add the flour and baking soda and mix just until combined and no clumps of dry flour is visible.
5. Pour the batter into a square pan and bake at 350F for 25-30 min or just until the bread turns golden and a toothpick inserted into the middle comes out clean. Let the bread cool in the baking tray.
6. While the bread is baking, make the frosting by melting the marshmallow in a pan over a low heat, then turn off the heat and add softened butter and peanut butter and mix until everything combined.
7. Once the banana bread is cooled, pour on the icing then enjoy :)
