To begin, preheat your oven to 425 and spray a muffin tin with non-stick spray.
To a large mixing bowl, add your butter, brown sugar, peanut butter and banana and whisk together till creamy. Add your milk till completely whisked in. Add your vanilla and eggs and whisk them in, till smooth.
To the same bowl, add your oat flour, baking soda and salt. Stir together till you have a smooth batter. Let your batter rest for 5-10 minutes (the oat flour will soak in the wet ingredients and thicken up the batter).
Fill each muffin cavity about 2/3 way full. Bake at 425 for 5 minutes. Then lower your oven temperature to 350 and bake for another 14-17 minutes, or till the muffins are springy to touch and a toothpick comes out clean.
Let cool entirely, then devour!
Stored in the fridge in an airtight container these muffins last about a week.