In a small cup or bowl, combine the flaxseeds and water. Stir well, then set aside for 10 minutes.
Preheat the oven to 350°F (175°C). Line an 8-inch (20-cm) square baking pan with parchment paper.
In a large bowl, combine the mashed bananas, soy milk, Date Syrup, lemon juice, vanilla, and flaxseed mixture. Mix well.
Whisk in the almond flour, oat flour, baking powder, and cinnamon until evenly mixed. Fold in the walnuts. If the batter is too thick, stir in a little more soy milk, 1 tablespoon at a time.
Transfer the cake batter into the prepared baking pan and smooth the top. Bake for about 30 minutes, or until lightly browned on top.
Remove the pan from the oven and set aside to cool completely. Once cool, invert the cake onto a plate. Cut the cake into squares and place each serving on a small dessert plate. Spoon some of the Blackberry-Almond Butter Sauce over each serving (if using).