Preheat and prep: Heat your oven to 350°F (175°C). Line a mini muffin pan with liners or coat lightly with nonstick spray.
Mix wet ingredients: In a medium bowl, whisk the mashed bananas, egg, yogurt, oil, vanilla, and sugar until smooth and well combined.
Combine dry ingredients: In a separate bowl, whisk the flour, baking soda, baking powder, salt, and cinnamon.
Bring it together: Add the dry ingredients to the wet ingredients.
Stir gently with a spatula until just combined. Do not overmix—some small lumps are okay.
Fold in chips: Sprinkle in the mini chocolate chips and fold lightly so they’re evenly distributed.
Portion the batter: Fill each mini muffin cup about 3/4 full. For small regular muffins, fill each cup just over halfway.
Bake: Mini muffins take about 12–15 minutes; small regular muffins take 15–18 minutes.
They’re done when the tops spring back lightly and a toothpick comes out clean or with a few moist crumbs.
Cool: Let muffins sit in the pan for 2–3 minutes, then transfer to a wire rack to cool completely. This helps them stay tender and not soggy.