Prepare the flax egg: In a small bowl, combine 1 tablespoon of flaxseed meal with 3 tablespoons of water. Let it sit for 5 minutes to thicken and become gelatinous.

Preheat the oven: Preheat your oven to 350°F (175°C) and grease or line a 9x5 inch loaf pan with parchment paper, leaving an overhang for easy removal.

Cream the butter and sugar: In a large mixing bowl, cream together the softened vegan butter and brown sugar until light and fluffy. A stand mixer or hand mixer works best, but you can also do this by hand.

Mash the bananas: In a separate bowl, mash the overripe bananas until smooth. A few small lumps are okay.

Combine wet ingredients: Add the mashed bananas, flax egg, plant-based milk, and vanilla extract to the creamed butter and sugar. Mix well until combined.

Combine dry ingredients: In a medium bowl, whisk together the flour, baking soda, baking powder, cinnamon, and salt.

Combine wet and dry: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix – a few lumps are okay.

Prepare the streusel: In a small bowl, combine the flour, brown sugar, vegan butter, and chopped walnuts. Using a fork or your fingers, cut the butter into the dry ingredients until the mixture resembles coarse crumbs.

Assemble and bake: Pour the batter into the prepared loaf pan. Sprinkle the streusel topping evenly over the batter.

Bake: Bake for 55-65 minutes, or until a toothpick inserted into the center comes out clean. If the streusel starts to brown too quickly, tent the loaf pan with foil.

Cool: Let the banana bread cool in the pan for 10 minutes before transferring it to a wire rack to cool completely. Slice and enjoy!