In a medium bowl or mason jar, mash the banana with a fork until mostly smooth with a few small chunks remaining.
Add the rolled oats, milk, chia seeds, vanilla extract, cinnamon, and salt to the mashed banana. Stir until everything is well combined.
Add 1 tablespoon of peanut butter to the mixture and stir it in completely. Take the remaining tablespoon and swirl it on top for a marbled effect.
Cover the bowl or jar tightly with a lid or plastic wrap. Refrigerate for at least 6 hours or overnight, allowing the oats to absorb the liquid and soften.
In the morning, give the oats a good stir. If they're too thick, add a splash of milk to reach your desired consistency.
Top with fresh banana slices, extra peanut butter drizzle, granola, or your favorite toppings before serving. Enjoy cold or microwave for 30-45 seconds if you prefer them warm.