Place the butter and sugar in a large mixing bowl and beat on medium-high speed until very pale and fluffy (about 3 to 5 minutes). Add one whole egg. Beat on medium-high speed until incorporated. Stir in the egg yolk and vanilla extract until combined. Scrape the bottom and sides of the bowl with a spatula, then add the flour, cornstarch, baking soda, and salt. Stir until thoroughly blended.