Preheat oven to 350°F. Prepare two 8-inch baking pans.

In mixer with whisk attachment, combine butter, oil, sugar, vanilla, and egg whites. Beat on medium for 30 seconds, then on high for 2-4 minutes until light and fluffy.

In a separate bowl, sift together flour, cornstarch, baking powder, and salt. Alternately add the flour mixture and milk to the sugar mixture, mixing on low to medium until just combined. Fold in zucchini and lemon zest by hand.

Pour batter into prepared pans and bake for 25-30 minutes.