Make the oats: In a medium bowl or storage container, mash 1 ripe banana until mostly smooth.Add ⅔ cup milk, ½ cup Greek yogurt, 1 tablespoon maple syrup, and stir until well until combined.Add 1 cup rolled oats and stir again until evenly mixed. Cover and refrigerate overnight, or for at least 4 hours.
Make the topping: Shortly before serving, stir together ½ cup Greek yogurt, 1 to 2 teaspoons maple syrup, and ½ teaspoon vanilla extract in a small bowl until smooth and creamy.
Assemble: Give the oats a good stir. If they seem too thick, add a splash of milk.Divide half of the crushed graham crackers and half of the banana slices between two jars or bowls.Spoon the oats on top, then add the remaining graham crackers and banana slices. Finish with the vanilla yogurt mixture.Serve immediately.
Substitutions
Banana → Frozen banana (thawed and mashed). Since banana is the main flavor, we don’t recommend other substitutes.