Preheat oven to 350 degrees F. Line or grease a muffin tin (enough for 10).
In a large bowl, mix the mashed banana, egg, coconut oil, apple cider vinegar, vanilla and almond milk until smooth.
In a separate bowl, mix together almond meal, flaxseed meal, rolled oats, collagen, baking soda, cinnamon, salt and espresso powder. Set aside.
Add dry ingredients to wet ingredients and mix until just combined. Fold in chocolate chips and walnuts
Fill the muffin cups up. These don't rise a ton, so I filled them most of the way. Bake 20-23 minutes until a toothpick comes out clean. Let cool in the pan for a couple minutes, then transfer to a rack to cool completely.