Prep the pan and oven. Heat the oven to 350°F (175°C).
Line a 12-cup muffin tin with paper liners or grease well.
Mix wet ingredients. In a large bowl, whisk banana, eggs, yogurt, honey, vanilla, and oil until smooth.
Add dry ingredients. Sprinkle in protein powder, flour, oats, cocoa, baking powder, baking soda, and salt. Stir gently until just combined. If the batter seems very thick, add 2–4 tablespoons milk to reach a thick but scoopable texture.
Fold in extras. Stir in mini chocolate chips or any add-ins from the variations below.
Fill the cups. Divide batter evenly among the 12 muffin cups.
They should be about 3/4 full.
Bake. Bake 15–18 minutes, until a toothpick comes out with a few moist crumbs and the tops spring back when lightly pressed.
Cool. Let muffins rest in the pan for 5 minutes, then transfer to a rack to cool completely. This helps them set and stay moist.