Add the milk, Greek yogurt, bananas, protein powder, cinnamon, and sea salt to a blender. If you're not using protein powder, add the maple syrup and vanilla extract instead. Blend until completely smooth.
Blend on high for 30–45 seconds, or until completely smooth and creamy.
Divide the rolled oats and chia seeds evenly between 4 jars or containers. Pour the banana milk evenly between the four jars.
Stir each jar well to make sure the oats and chia seeds are evenly distributed, and there are no dry pockets at the bottom.
Cover and refrigerate for at least 2 hours, but preferably overnight. The oats and chia seeds will absorb the liquid as they sit, creating a thick, creamy pudding-like consistency.
Before serving, top with fresh banana slices, a spoonful of Greek yogurt or peanut butter, and crushed vanilla wafers or graham crackers, if desired.
Enjoy cold, or let the oats sit at room temperature for a few minutes before eating.