Preheat the oven to 350°F. Generously grease a bundt pan.
In a large bowl, combine the cake flour, sugar, baking powder, baking soda, salt, and cornstarch, and whisk to combine. Set aside.
Place the aquafaba in a small bowl. Using a hand mixer, beat until foamy throughout, about 1-2 minutes.
Add the beaten aquafaba, sour cream, oil, water, vanilla, and apple cider vinegar to the dry ingredients. Use the hand mixer to beat the mixture until well-combined. Do not over mix.
Make the topping. Combine the ¾ cup granulated sugar and 3 teaspoons of cinnamon in a small bowl, and stir to combine.
Coat the greased bundt pan with about 2-3 tablespoons of topping, turning the pan so that the mixture covers the whole inside surface. After coating, pour any loose topping back into the topping bowl.
Pour about one-third of the cake batter into the bundt pan. Sprinkle one-third of the remaining cinnamon-sugar mixture evenly over the batter. Repeat this process two more times.
Bake the cake for about 1 hour, then insert a toothpick to check for doneness. The toothpick should come out clean or with a couple of crumbs.
Allow the bundt to cool completely. Carefully loosen the edges of the cake with a knife, then invert the pan over a plate and gently let the cake come out of the pan. Enjoy!