Preheat oven to 325°F (163°C). Grease three 8-inch round cake pans, line with parchment paper circles, and grease the parchment.
In stand mixer bowl, whisk together flour, cornstarch, sugar, baking powder, baking soda, and salt for 30 seconds. Switch to paddle attachment.
Add softened butter to dry ingredients on low speed, mixing for 2-3 minutes until mixture resembles coarse sand.
In a separate bowl, whisk egg whites until frothy, then add buttermilk, oil, vanilla extract, almond extract, and vinegar until combined.
Pour half the wet mixture into butter-flour mixture on low speed for 30 seconds. Scrape bowl, add remaining wet ingredients, then beat on medium speed for exactly 2 minutes.
Divide batter evenly between three prepared pans (about 720g per pan). Tap pans gently on counter to release air bubbles.
Bake for 30-35 minutes at 325°F until tops spring back when pressed and toothpick comes out clean. Check at 28 minutes.
Cool cakes in pans for 10 minutes, then run knife around edges, invert onto cooling racks, remove parchment, and cool completely (1-2 hours).
Beat cream cheese on medium speed for 2 minutes until smooth. Add butter and beat 2-3 minutes until fluffy.
Add powdered sugar one cup at a time on low speed. Add vanilla and salt, then beat on medium-high for 3-4 minutes until light and fluffy.
Place first cake layer on serving plate, spread 1 cup frosting on top. Add second layer, frost, then add final layer. Apply thin crumb coat and refrigerate 15-20 minutes.
Apply final layer of frosting using offset spatula for smooth finish. Decorate as desired with frosting swirls, berries, or sprinkles.