Preheat oven to 350°F. Line a muffin tin with liners or lightly grease.
Mix dry ingredients: In a bowl, whisk together oat flour, cocoa powder, baking soda, and salt.
Mix wet ingredients: In another bowl, whisk together maple syrup, mashed banana, Greek yogurt, vanilla, egg, and milk.
Combine: Gradually add the wet ingredients to the dry ingredients, stirring until just combined.
Fold in chocolate chips, if using.
Fill muffin cups evenly with the batter.
Bake for 15–18 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Cool for a few minutes before enjoying!