Preheat the oven to 375°F. Line a 12-cup muffin pan with liners or lightly grease.
Blend the dry ingredients. Add the oat flour, cocoa powder, baking soda, and salt to a blender. Blend briefly until evenly combined.
Add the wet ingredients. Add the egg, egg yolk, oat milk, mashed bananas, maple syrup, melted coconut oil, and vanilla extract. Blend until smooth, stopping to scrape down the sides as needed. Avoid overmixing.
Portion. Divide the batter evenly among the prepared muffin cups.
Top with chocolate. Sprinkle the mini semisweet chocolate chips evenly over the tops of the muffins.
Bake. Bake for 15–18 minutes, until a toothpick inserted in the center comes out mostly clean.
Cool. Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool to your liking. Serve and enjoy!