Prepare two 8 inch baking pans with baking spray lined with parchment paper.
Preheat oven to 350 ℉.
In a medium bowl, whisk together flour, cornstarch, salt, and baking powder. Set aside.
In the bowl of a standing mixer with paddle attachment, or a large both with a hand held mixer, cream butter and sugar for 3-4 minutes. It should be light and fluffy.
Slowly add the egg whites, one at a time and mix fully after each addition, scraping down the sides of the bowl with a spatula as necessary.
Mix in oil and vanilla extract until incorporated.
Alternate adding the flour mixture and the whole milk, starting and ending with the dry ingredients (three additions of flour and two of milk).
Do not over mix, you want to stop as soon as all the flour is incorporated.
Divide the cake batter evenly between the prepared cake pans, smoothing the tops with a spatula.
Bake in preheated oven 25-30 minutes, or until you can poke the middle of the cakes with a toothpick and it comes out mostly clean or with only a few crumbs on it.
Let cool in the pan for 10 minutes and then let cool completely on a wire rack before frosting.