Mash the banana in a bowl until smooth.
Whisk in the milk, maple syrup, and vanilla.
Stir in the chia seeds and let sit for 5 minutes, then stir again to break up clumps.
Cover and refrigerate for at least 2 hours or overnight until thickened.
Before serving, stir in the yogurt if using, then top with banana slices, granola, or whipped cream.