Preheat the oven to 425°F (218°C). Put muffin papers in four compartments of a muffin tin and mist them with cooking spray.
Mash the banana until completely smooth with a potato masher or fork.
Combine the peanut flour, Swerve, milk powder, baking powder, cocoa powder, and ground flax in a medium-sized bowl. Combine the banana, eggs (or egg whites), and oil in a second bowl. Dump your wet ingredients into your dry ingredients, and stir until combined.
Put the muffin batter into the oiled muffin papers that are in your muffin tin. If the ingredients were weighed and measured correctly, the paper liners should be filled to the top.
Top the muffins with the chopped peanuts, pressing them into the batter slightly.
Bake for 13-14 minutes about ⅓ of the way from the top of the oven. When finished baking, a toothpick inserted into the center of the muffin will come out clean or with crumbs. If you undercook the muffins, the middles will sink as they cool.
After removing them from the oven, let the muffins cool for a few minutes before carefully taking them out of the muffin pan. Let them come to room temperature on a cooling rack before putting them in the fridge. For the best texture, let the muffins chill completely before eating.