Preheat oven to 180°C (350°F) and prepare a muffin mold with muffin liners.
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 and baking powder. Stir the ingredients together while pouring in the milk gradually. Start with about 45 ml (3 tbsp) and add more in case the batter is too thick, until it reaches a smooth consistency.
Mix in the chocolate chips.
Fill ⅔ of each muffin liner with the batter, sprinkle with more chocolate chips and bake in hot oven for 20 to 25 minutes. Make sure you do not overbake the muffins; they are ready when a toothpick inserted into the center comes out clean.
Remove yogurt muffins from the oven and let cool over a baking rack for about 15 minutes before eating.