In a medium or large mixing bowl, mash bananas. Whisk in egg, then add peanut butter and honey. Stir well and then add milk and vanilla.
On top of the liquid ingredients, add oats, cinnamon, baking powder, and salt. Stir dry ingredients together on top and then mix dry ingredients all the way in with liquid ingredients.
Spray muffin pan with cooking spray and distribute batter evenly among tins. Top each muffin cup with a banana slice if desired. Bake for 18-20 minutes, until mostly set. Top each muffin with a light spread or drizzle of peanut butter, if desired (you can briefly heat it to thin the consistency for drizzling).
Store in a air-tight container or gallon-size zipper bag in the fridge for up to a week or freezer for up to 3 months.
I've also tried making these muffins with powdered peanut butter instead of regular peanut butter (I buy organic PBFit at Costco or on Amazon for a similar price!) and couldn't tell a difference! I subbed in 1/4 cup powdered peanut butter and added it with the dry ingredients. I still used regular peanut butter for the drizzle on top.