Preheat oven to 350°F and line a muffin tin with paper liners.
In a large bowl, mix oat flour, protein powder, cocoa powder, baking powder, baking soda, and salt.
In a separate bowl, whisk together egg whites, milk, Greek yogurt, maple syrup, and vanilla extract.
Pour wet ingredients into dry ingredients and stir until just combined; do not overmix.
Fold in mini dark chocolate chips if using.
Divide batter evenly into 8 muffin cups.
Bake for 18-20 minutes or until a toothpick comes out clean.
Allow muffins to cool before serving.