Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
Mash the bananas in a mixing bowl until mostly smooth.
Add the egg and vanilla extract. Mix until combined.
Stir in the protein powder, baking powder, salt, and cinnamon if using.
Add the oats and mix until a thick dough forms.
Fold in the mini M&M’s.
Let the dough sit for 5–10 minutes so the oats can absorb some of the moisture.
Divide the dough into 5 large cookies on the baking sheet. Press them down slightly because they will not spread much.
Bake for 10–13 minutes, or until the edges are set and the tops look cooked through.
Let the cookies cool for at least 10 minutes before moving. They firm up as they cool.