Pre-heat oven to 350 degrees. Lightly oil a mini muffin/cupcake tin.
In a medium-sized bowl, whisk together whole wheat flour, old-fashioned oats, and baking powder.
In the bowl of a stand mixer or in a large bowl with a hand mixer, cream together peanut butter and mashed banana. Mix in eggs.
Mix in flour mixture, alternating with water, until just combined.
Fill mini muffin tin cavities about ¾ of the way with batter.
Bake for 6-8 minutes, until a toothpick inserted in the middle of a cupcake comes out clean.
Let cool in tin for a few minutes before moving to wire racks to cool completely.