Blend your buckwheat groats into a flour in a high speed blender. It takes about 1 1/4 cup of groats to make 1 cup of flour. Measure your flour after grinding it. *It is difficult to get a fine enough flour in a food processor, so I suggest using a blender.
Preheat oven to 350º F. Line a standard muffin pan with parchment paper liners and set aside.
In a large mixing bowl, combine the buckwheat and almond flours with the baking powder, sea salt, and cinnamon. Stir to combine the dry ingredients.
In a separate large bowl, mash the banana until almost smooth.
Add the eggs, coconut oil, and vanilla to the mashed banana and stir to combine.
Add the dry ingredients and the shredded zucchini into the bowl with the wet ingredients. Stir until no dry ingredients remain.
Fold in chopped walnuts and chocolate chips, or other additions, if using.
Divide the batter equally between the 12 muffin papers. If desired, top each muffin with some chopped walnuts and/or extra chocolate chips.
Bake the muffins for 24-26 minutes, or until lightly golden brown and fully set in the centers.
Allow the muffins to cool for about 5 minutes, and then pop them to the sides to prevent them for steaming while cooling to room temperature.