Warm the base: Add oats, milk, cinnamon, and a pinch of salt to a small saucepan. Stir and bring to a gentle simmer over medium heat.
Mash the banana: While the oats heat, mash the banana in a bowl with a fork until mostly smooth.
Cook the oats: Reduce the heat to low and cook the oats for 3–5 minutes, stirring often, until creamy and slightly thickened.
Stir in the banana: Add the mashed banana and vanilla extract.
Cook for 1–2 more minutes to warm through. Taste—if you want more sweetness, add a little maple syrup or honey.
Add protein powder: Remove the pan from heat. Sprinkle in the protein powder gradually, whisking or stirring well to avoid clumps.
If the mixture gets too thick, add a splash more milk.
Finish with nut butter: Stir in peanut or almond butter until melted and well combined.
Serve and top: Pour into a bowl. Add chopped walnuts or pecans, banana slices, and any extras like chia seeds or a drizzle of nut butter.