Preheat oven to 350 degrees F. Line two 8-inch cake pans with rounds of parchment and grease the sides with cooking spray or flour and butter. Set aside.
To a medium bowl, whisk together the all-purpose flour, corn starch, baking powder, baking soda and salt. Set aside.
In a large measuring cup, measure out the milk and whisk in the sour cream until mostly smooth; add the egg whites. Whisk together and set aside.
To the bowl of a stand-up mixer (or using a large bowl and an electric hand mixer), add the butter, sugar and vanilla extract or vanilla paste. Beat until fluffy, about 2 to 3 minutes. Pour in the vegetable oil and mix in.
Next, rotating between the milk/egg white mixture and the dry ingredient, add, ending and beginning with the liquid mixture. Beat for an additional 20 seconds, until everything is combined and cohesive. Give it a manual stir with a spatula just to ensure all of the flour at the bottom of the bowl is properly combined.
Divide the batter amongst the two cake pans and using a spatula, even out the top and spread the batter to the edges of the cake pans. Transfer to the oven to bake for about 30 to 35 minutes, until a skewer inserted into the center comes out clean. Cool the cakes in the pan for 5 minutes and then carefully invert onto cooling racks. Make sure they're completely cooled before adding the buttercream.