Preheat oven to 350°F (175°C).
Lightly grease or line a mini muffin pan.
In a bowl, mash the sweet potato and banana until smooth.
Add peanut butter, cocoa powder, flour, and baking powder, and mix until well combined.
Fold in chocolate chips, if using.
Scoop the batter into the mini muffin pan, filling each cup about ¾ full.
Bake for 14 minutes. The brownies should be a bit fudgy.
Let cool completely before removing from the pan — they’ll firm up as they cool.