Preheat the oven to 350℉ and grease a 9x13 inch pan.
Mash the bananas in a medium bowl and set aside. I just use my stand mixer for this, then set them aside in a bowl.
In a large measuring cup, add the soy milk, apple cider vinegar and ground flaxseeds. Whisk to combine, then set aside.
In medium bowl, add the flour, baking powder, baking soda, cinnamon and salt and whisk to combine. Set aside.
In a stand mixer with the paddle attachment or in a large bowl using a handheld mixer, beat the vegan butter for about a minute until creamy. Now add the sugars and beat for 2 minutes until combined. Add the vanilla and mashed bananas and mix in.
Alternate adding the milk/flaxseed mixture and the dry ingredients, in about three additions, mixing until each addition is just incorporated. Do not over mix.
Transfer the cake batter to the pan and bake for 45-50 minutes, or until a toothpick inserted in the center comes out clean.
Remove from oven and let cool in a wire rack. Let it cool completely before frosting. I usually stick it in the refrigerator to speed this up.