Preheat oven to 350°F (180°C) and prepare a muffin mold with muffin liners. Make sure you grease the liners to prevent the cupcakes from sticking.
In a large bowl, mash the banana then combine with Greek yogurt, egg, maple syrup, vanilla extract and a pinch of salt.
Stir well until you get a smooth mixture.
Add into the wet mixture oat flour, protein powder and baking powder.
Stir the ingredients together while pouring in the milk gradually. Start with about 3 tbsp (45 ml) and add as much as ⅓ cup (80 ml) of milk until the batter becomes smooth, but not too liquid. This depends on protein powder used.
Fold through the chocolate chips or any mix-in you'd like.
Fill ⅔ of each muffin liner with the batter, sprinkle with more chocolate chips and bake in hot oven for 20 to 25 minutes. Do not overbake them. They are ready when a toothpick inserted into the center comes out clean.
Remove your protein muffins from the oven and let cool over a baking rack for about 15 minutes.