Make the crumble. In a bowl, combine the flour, sugars, cinnamon, and salt. Mix to combine. Add the melted butter and stir with a fork until sandy and crumbly. Place in the fridge to set while you prepare the cake.
Make the vegan buttermilk. Combine the plant milk and apple cider vinegar. Set aside for at least 5 minutes, or until it curdles.
Mash the bananas. If frozen, microwave to thaw, then mash.
Preheat the oven to 325°F. Line an 8x8 baking pan with parchment. Alternatively, you can use a springform round pan with a parchment layer just on the bottom, as I did here.
In a large bowl, combine the sugars and butter. With an electric mixer, beat until fully combined and fluffy, about 3-5 minutes of whipping.
In a medium bowl, whisk together the dry ingredients: flour, Baking Powder, Baking Soda, and Salt.
Add the vanilla extract, banana, sourdough discard (or vegan yogurt), and vegan buttermilk to the butter mixture. Whisk until fully combined and light, pale, and a little fluffy.
Pour the dry ingredients over the wet ingredients. Using a spatula, fold to combine. Fold until just combined, making sure to scrape the bottom to fully incorporate the wet and dry. It may be a little lumpy, and that's okay! Just make sure there aren't any separate dry and wet spots. The batter will seem thick and a little dry, but it bakes up moist!
Let the batter rest. Stir together the brown sugar and cinnamon to make your cinnamon mixture.
Pour half the batter into the pan. Smooth the batter out using an offset spatula. Sprinkle the cinnamon mixture over the batter.
Add the rest of the batter, smoothing it over with a spatula. Some of the cinnamon layer may marble/ mix into the top layer of batter. That's okay!
Sprinkle the crumble on top. Press it lightly into the batter with your hands, as there's a lot of it and we want it to stick!
Bake for 20 minutes. Rotate the pan. Bake for another 20 minutes, or until a toothpick inserted into the center of the cake comes out relatively clean. There is crumble on the cake, so that will most likely get on your toothpick.
Allow to cool for 20-30 minutes before taking it out of the pan. Cool completely on a wire rack before cutting.