Preheat oven to 350 degrees F.
Line an 8x8 baking pan with parchment paper, letting the ends overhang so you can use them as handles later. Or, if you don't want to use parchment, lightly spray the baking pan with cooking spray.
In a large mixing bowl, stir together the almond butter, coconut sugar, aquafaba, vanilla, and smashed bananas.
In a small mixing bowl, whisk together the flour, baking soda, and salt.
Gently stir the dry ingredients into the wet ingredients until just combined.
Pour the batter into the prepared baking pan.
Sprinkle the slivered almonds, if using, all over the top of the batter.
Bake for 40-45 minutes until a toothpick inserted in the center comes out clean.
Let cool in the pan on a wire rack for 10 minutes. Using the parchment "handles" lift out the banana bread cake and continue to let it cool on the wire rack.
If you are using the Optional Almond Butter Frosting: Add all ingredients to a small bowl and stir until well combined. Add a touch more liquid if you want it thinner or add a touch more sugar if you want it thicker.
Cut into 16 squares and enjoy!
*Nutrition facts calculated without the optional Almond Butter Frosting.