Preheat oven to 350 degrees F and either line a mini muffin pan with liners or grease each spot with coconut oil (that's what I did).
In a small bowl, mash banana until smooth.
Add egg in and whisk.
Add oat milk, vanilla extract and cinnamon and whisk until well combined (I added the cinnamon to the wet ingredients but you can wait to add it into the dry ingredients).
In another bowl, add the coconut flour, buckwheat flour, baking powder and sea salt. Mix together.
Add the wet ingredients into the dry ingredients. Mix until well combined. You might need to add a bit more liquid, depending on the consistency. You don't want it to be sticky or runny, something in between! I added about 2-3 tbsp. more of oat milk.
Fold in the chocolate chips.
Fill each muffin cup.
Bake for about 15-20 minutes, depending on your oven. Make sure to check on them throughout the cooking time. Put a toothpick through one to check if it's done. If it comes out clean, you're good to go!