Preheat oven to 350°F (180°C). Grease or line a 9x5-inch loaf pan with parchment paper.
Prepare flax egg: Mix 1 tablespoon ground flaxseed with 3 tablespoons hot water. Let sit 5–10 minutes until thickened.
Mix dry ingredients: In a medium bowl, whisk together flour, baking soda, baking powder, salt, and cinnamon (if using).
Mix wet ingredients: In a large bowl, whisk melted vegan butter, brown sugar, and vanilla. Add flax egg and vegan yogurt; whisk until smooth.
Mash bananas: In a separate bowl, mash bananas thoroughly with a fork or potato masher until smooth.
Combine wet and dry: Add mashed bananas to the wet mixture. Gradually fold in dry ingredients until just combined—do not overmix. Batter should be thick with some streaks of flour remaining.
Add mix-ins: Fold in chopped nuts and/or chocolate chips if using.
Bake: Pour batter into prepared loaf pan. Bake for 50–60 minutes, tenting with foil if top browns too quickly. Check doneness with a toothpick.
Cool: Let cool in pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.