In a small bowl, add the ground flaxseed and cold water. Stir it lightly and set it aside for 10 minutes to thicken.
In the meantime, preheat the oven to 350°F and lightly oil a bread loaf pan.
Mix the buckwheat, tapioca, and oat flours together in a medium sized bowl. Stir in the baking soda, salt, and cinnamon. Set aside.
In a large bowl, add the bananas and mash them well. Then add in the coconut sugar, vanilla extract, oil, and flax eggs. Stir well.
Next, pour the flour mixture into the banana batter, stirring until all ingredients are mixed thoroughly. Add in any chopped nuts or chocolate chips.
Pour the batter into the bread pan and bake it on the middle oven rack for 1 hour, or until an inserted toothpick comes out clean.
Allow the bread to cool completely before slicing.