Prep the oven and pan. Preheat to 350°F (175°C).
Line a 12-cup muffin tin with paper liners or lightly grease with oil.
Mash the bananas. In a large bowl, mash ripe bananas until mostly smooth. Some small lumps are fine.
Whisk the wet ingredients. Add maple syrup, Greek yogurt, olive oil, eggs, and vanilla. Whisk until well combined and creamy.
Combine the dry ingredients. In a separate bowl, whisk flour, cocoa powder, baking soda, baking powder, salt, and espresso powder.
Bring it together. Add the dry ingredients to the wet.
Stir gently with a spatula until just combined. If the batter seems very thick, fold in 2–4 tablespoons milk to loosen. Don’t overmix.
Fold in the chocolate chips. Reserve a few for the tops if you like bakery-style looks.
Fill the cups. Divide the batter evenly among the 12 muffin cups.
Top with the reserved chips.
Bake. Bake for 16–20 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs but no 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 texture.
Enjoy. Serve warm or at room temperature. They’re great plain or with a smear of nut butter.