Preheat the oven to 350°F and lightly grease or spray a 12-cup muffin tin.

In a small bowl, combine the flaxseed and warm water and set aside to thicken for 5 minutes.

In a medium bowl, whisk together the flour, cinnamon, nutmeg, baking powder, baking soda, and salt.

In a large bowl, whisk together the mashed banana, almond milk, maple syrup, oil, apple cider vinegar, and vanilla. Stir in the flaxseed mixture.

Add the dry ingredients to the wet ingredients and mix just until combined. Do not overmix. Fold in the chocolate chips, if using. Use a 1/3 cup measuring scoop to evenly divide the batter into the muffin tin.

Bake for 18 to 22 minutes, or until a toothpick comes out almost clean. Let cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.