Preheat oven to 350F and line a standard muffin tin with cups.
In a mixing bowl, combine the ground flaxseeds with 3 tablespoons of water and mix well. Allow to set for 5-10 minutes, until it gels together.
Add in the buckwheat flour, cinnamon, salt, coconut oil, mashed bananas, and water. Stir until a uniform batter is formed.
Add in the baking soda and vinegar. Mix quickly to distribute throughout the batter.
Gently fold in the walnuts or other add-ins, then use a 1/4 cup to scoop the water into the paper muffin liners, filling them about 3/4 of the way full.
Bake at 350F for about 20 minutes, until the centers are firm and the tops are starting to crack.
Allow to cool for 5 minutes in the pan, then transfer the muffins to a wire rack to cool completely.
These muffins can be stored at room temp for a couple of days, but for best shelf life, store them in a sealed container in the fridge.