Prep the oven and dish: Heat oven to 350°F (175°C). Lightly grease a 16–20 oz oven-safe ramekin or small baking dish.
For meal prep, use an 8x8-inch pan and double the recipe.
Mash the banana: In a bowl, mash the banana until mostly smooth. A few small lumps are fine.
Whisk in wet ingredients: Add egg, Greek yogurt, vanilla, and maple syrup (if using). Whisk until creamy.
Add dry ingredients: Stir in oats, protein powder, baking powder, cinnamon, and salt.
If the batter feels very thick, add 1–2 tablespoons milk to loosen. It should be like thick muffin batter.
Fold in extras: Gently fold in chocolate chips, walnuts, or blueberries if you like.
Pour and smooth: Transfer to the prepared dish and smooth the top.
Bake: Bake 18–25 minutes for a single serving, or 25–30 minutes for an 8x8 pan, until the top is set and a toothpick comes out mostly clean with a few moist crumbs.
Rest and serve: Let cool 5 minutes to set. Enjoy warm as is, or top with a little peanut butter, yogurt, or a drizzle of maple.