Heat oven to 350°F. Prepare two 8in baking pans.
In a stand mixer with paddle attachment, combine butter, sugar, and vanilla. Beat on medium speed for 30 seconds, then on high for 2-4 minutes. You are looking for a light and fluffy sugar mixture. Add egg whites and mix until just incorporated.
In separate bowl, sift together flour, cornstarch, baking powder, and salt. Alternating as you go, add the flour mixture and milk to sugar mixture. Mix on low until just combined. Remove bowl from mixer and zucchini, folding in by hand.
Pour batter into prepared pans and bake 25-30 minutes.
Remove from oven when an inserted toothpick is removed MOSTLY clean, a few crumbs are good. You do not want to overbake this cake.
Cool in pans about 15 minutes then invert onto a cooling rack.
Cool completely before frosting and stacking.