Preheat oven to 375 degrees F and grease a mini muffin tin.
Stir the chia seeds into the milk in a medium bowl.
Add the rest of the ingredients, including the mashed banana, to the bowl with the milk.
Fill the prepared mini muffin tin to just under the edge of each cup, about 1 1/2 tablespoons of batter each.
Bake for 12-14 minutes or until a cake tester inserted into the center of a muffin comes out clean. Transfer to a wire rack to cool.
Store in an airtight container for up to 5 days in the fridge. Warm slightly before serving. Or, store in a zip-top freezer bag with as much air removed as possible for up to 3 months. Thaw overnight in the fridge or for 15-30 seconds in the microwave.
Notes
Make 9 full-size muffins by using a standard muffin tin and baking for 20-22 minutes.
Add 1/3 cup mini chocolate chips or raisins to the batter if desired.
Use melted and cooled butter in place of the oil if desired.
Use dairy or nondairy milk.
Use very ripe bananas with brown spots for the best flavor and sweetness.