Cream butter and sugar till smooth. Stir in flour and cornstarch. Mix well, then cover and chill for at least an hour. Roll into 1" balls and place on parchment lined cookie sheets. Bake at 325° for about 15-18 minutes. Cool completely, then frost with lemon cream cheese frosting.
For frosting: Beat butter, cream cheese, and salt till smooth and creamy. Add lemon zest and enough powdered sugar to make a spreadable frosting. Frost the tops of the cookies.