Preheat oven to 350 degrees F. Cream butter and sugars together until fluffy and light in color; blend in eggs and vanilla. Mix in flour, cornstarch, baking soda and salt. Stir in chocolate chips. Drop by tablespoons onto a greased baking sheet and bake for 8-10 minutes, until slightly golden brown around the edges. (The tops will not be brown, but that is okay.) Let cool for 5 minutes before removing from baking sheet.