Prep the pan and oven: Preheat the oven to 350°F (175°C).
Line a 12-cup muffin tin with paper liners or lightly grease the cups. Liners help with portion control and clean release.
Mash the bananas: In a large bowl, mash the bananas until mostly smooth. A few small lumps are fine and add texture.
Whisk in the wet ingredients: Add the egg, Greek yogurt, oil, sugar, and vanilla.
Whisk until the mixture looks creamy and well combined.
Combine the dry ingredients: Sprinkle the flour, cocoa powder, baking powder, baking soda, and salt over the wet mixture. Gently fold with a spatula until just combined. Do not overmix.
Add optional chips: If using mini chocolate chips, fold them in now.
Reserve a few to sprinkle on top for a bakery-style look.
Portion the batter: Divide evenly among the 12 muffin cups. The cups should be about two-thirds full.
Bake: Bake for 12–15 minutes, or until the tops spring back lightly and a toothpick comes out with a few moist crumbs (not wet batter).
Cool: Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely. This helps set the crumb and keeps them tender.