Mash the banana: In a jar or bowl, mash the ripe banana with a fork until mostly smooth. A few small chunks are fine.
Stir in the wet ingredients: Add the milk, peanut butter, vanilla, and sweetener if using.
Whisk or stir until the peanut butter is well incorporated.
Add dry ingredients: Mix in the oats, chia seeds, salt, and cinnamon. Stir until everything is evenly combined.
Adjust consistency: If the mixture looks very thick, add a splash more milk. It should be pourable but not watery.
Cover and chill: Seal the jar or cover the bowl.
Refrigerate for at least 4 hours, but overnight gives the best texture.
Stir and top: In the morning, give it a good stir. Add more milk if you like it looser. Top with sliced banana, a drizzle of peanut butter, or a sprinkle of nuts.
Enjoy cold or warm: Eat straight from the fridge, or warm gently in the microwave for 30–60 seconds, adding a splash of milk if needed.