Preheat your oven to 350F and line a standard muffin tin with 12 parchment cups.
In a medium mixing bowl, combine the ground flax or chia seeds with the 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 or milk, and stir until a uniform batter is formed. Add in the baking soda and vinegar, and 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 batter 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 temperature for a couple days, but for best shelf life, store them in a sealed container in the fridge.
These muffins don't rise as much as a traditional gluten-based muffin, so they will be on the smaller side. You could probably double the amount of batter in each cup to create 6 larger "bakery style" muffins, if you prefer! Just increase the baking time, as well, to make sure they are cooked thoroughly.