Preheat oven 375°F. Whip butter with an electric mixer until fluffy. Stir in sugar, flour and cornstarch, and beat on low for about one minute. Once smooth, beat on high for 3-4 minutes. Drop cookies by the tablespoonful onto an ungreased sheet pan or parchment-lined pan. Bake for 12-15 minutes, or until lightly golden on the edges.