Preheat the oven to 375°F (190°C) and lightly grease a 12-cup muffin pan.
In a large bowl, mash 2 ripe bananas with a fork until mostly smooth (small lumps are okay). Add 1 cup Greek yogurt, ⅓ cup milk, ¼ cup honey, 1 large egg, and 1 teaspoon vanilla extract. Whisk until smooth and creamy.
Add 1½ cups rolled oats, 1 teaspoon cinnamon, and 1 teaspoon baking powder. Stir well until everything is evenly mixed and no dry oats remain.
Stir in ¼ cup chopped walnuts, pecans, raisins, or dark chocolate chips. You can use one or mix two together (¼ cup for each).
Spoon the mixture into the muffin pan, filling each cup almost to the top. If you like, place a thin banana slice on top of each cup.
Bake at 375°F (190°C) for 20–23 minutes, until the tops are lightly golden and the centers feel set. Let the cups cool in the pan for about 15 minutes, then run a small knife around the edges and lift them out.
