Mash the banana. In a small bowl, mash the ripe banana until mostly smooth.
A few small chunks are fine and add texture.
Warm the base. In a small saucepan, add the oats, milk, cocoa powder, and a pinch of salt. Whisk the cocoa into the milk so it dissolves evenly before turning on the heat.
Cook the oats. Bring to a gentle simmer over medium heat, stirring frequently. Cook for 4–6 minutes, or until the oats are creamy and slightly thickened.
Stir in banana and vanilla. Reduce heat to low and fold in the mashed banana and vanilla extract.
Cook another 1–2 minutes, stirring, to warm through and meld flavors.
Add protein powder off heat. Remove the pan from the heat. Whisk in the protein powder until fully blended. If the mixture gets too thick, add a splash more milk to loosen.
Sweeten to taste. Taste the oatmeal.
If you want more sweetness, stir in maple syrup or honey 1 teaspoon at a time.
Serve and top. Transfer to a bowl. Add your favorite toppings—sliced banana, a small drizzle of peanut butter, a few chocolate chips, or a sprinkle of nuts for crunch.