Preheat your oven to 350°F (175°C). Line a muffin tin with 6 paper liners or lightly grease it.
In a large bowl, combine the oats, cinnamon, baking powder, and salt.
In another bowl, whisk the eggs, mashed banana, Greek yogurt, milk, and vanilla extract until smooth.
Add the wet mixture to the dry ingredients and stir until everything is fully incorporated. If you’d like, fold in any optional mix-ins like chocolate chips or nuts.
Spoon the mixture evenly into your muffin tin cups, filling each about 3/4 full.
Bake for 18-22 minutes, or until the tops are firm and a toothpick inserted comes out clean. They may be a little golden on top.
Let the cups cool for about 10 minutes in the pan, then transfer them to a wire rack to cool completely. For the best results, refrigerate for at least an hour before serving to let them set fully.