Heat oven to 350℉/177℃.
Spray 3 eight-inch cake pans with baking spray. Set aside.
Toss apples with 3 Tablespoons corn starch. Set aside.
In a medium bowl, whisk flour, 3 Tablespoons corn starch, baking powder, baking soda, salt, and cinnamon. Set aside.
In a large bowl, whisk eggs and sugars.
Add applesauce, buttermilk, vegetable oil, and vanilla.
Whisk until thoroughly combined.
Add dry ingredients.
Stir until just mixed.
Fold in apples.
Divide evenly into prepared pans.
Bake for 25 minutes, or until cake tests done.