To a small mixing bowl, add your oat flour, cocoa powder, baking powder, baking soda and salt and whisk them together thoroughly.
To a large mixing bowl, add your butter and sugar and beat them together with a hand mixer till creamy. Add your banana and mix together till smooth. Add your milk, vanilla and eggs and briefly blend in.
Add your dry ingredients to your wet ingredients and mix together till smooth. Preheat your oven to 350 and line a muffin tin (or spray with non-stick spray). Let your batter rest for about 10 minutes, while your oven preheats.
Fill each muffin tin about 2/3-3/4 way full with batter. Bake for 18-21 minutes, or till fluffy to touch and a toothpick inserted comes out clean.
Let cool, then devour!
Stored in an airtight container in the fridge these muffins last about a week.