Preheat oven to 180°C / 350°F. Line a muffin tin with cases or grease lightly.
Blend egg, banana, yogurt, maple syrup, vanilla, and spinach until smooth and green.
In a large bowl, whisk together oat flour, cacao, baking powder, baking soda, and salt.
Pour wet mixture into dry, stir gently until just combined. Fold in chocolate chips.
Divide batter into muffin cups, filling about 3/4 full.
Bake for 16–20 minutes, or until a toothpick comes out with just a few moist crumbs.
Let cool fully before serving so they set nicely.