Prep the pan: Heat oven to 350°F (175°C). Line a 12-cup muffin tin with liners or lightly grease.
Mix dry ingredients: In a large bowl, whisk oat flour, protein powder, cocoa, sugar, baking powder, baking soda, and salt until no clumps remain.
Add wet ingredients: Whisk in eggs, Greek yogurt, milk, oil, and vanilla.
Stir until just combined. The batter will be thick but scoopable.
Fold in chocolate: Gently fold in the mini chocolate chips. Do not overmix.
Fill the cups: Divide batter evenly among the muffin cups.
Sprinkle a few extra chips on top if you like.
Bake: Bake 15–18 minutes, or until the tops are set and a toothpick comes out with a few moist crumbs (not wet batter).
Cool: Let muffins cool in the pan for 5 minutes, then transfer to a rack to finish cooling.