Preheat the oven to 350°F (175°C) and line a 9x5-inch loaf pan with parchment paper, leaving some overhang on the long sides so you can lift the finished loaf out easily.

In a small bowl, whisk the ground flaxseed with the warm water and let it sit for about 5 minutes, until it thickens into a gel. In a large bowl, mash the bananas with a fork until mostly smooth with a few lumps left.

Stir the melted vegan butter, coconut sugar, and vanilla extract into the mashed bananas until evenly combined, then stir in the flaxseed gel. The sugar won't fully dissolve, and that's fine.

In a separate bowl, whisk together the flour, baking soda, salt, and 1 teaspoon of the cinnamon. Pour the dry ingredients into the wet mixture and fold with a spatula just until no dry streaks remain. Do not overmix, a few small streaks of flour are better than a tough loaf.

Fold in the walnuts or chocolate chips if using, then scrape the batter into the prepared pan and smooth the top. Sprinkle the cinnamon sugar topping evenly over the surface.

Bake for 55 to 65 minutes, until a toothpick inserted in the center comes out clean or with just a few moist crumbs. If the top is browning too quickly, tent the pan loosely with foil after about 40 minutes.

Let the loaf cool in the pan for 10 minutes, then lift it out using the parchment and transfer it to a wire rack to cool completely before slicing. It slices much cleaner when fully cool.