Cream together butter, shortening, brown sugar and granulated sugar for 2 minutes. Add eggs, one at a time, mixing well between additions. Add vanilla.
In a seperate bowl, combine flour, cornstarch, baking soda and salt. Add dry ingredients to creamed mixture and mix until just combined. Stir in chocolate chips.
Line two large baking sheets with parchment paper. Scoop out dough using a cookie scoop. Flatten slightly. The cookies are going to be frozen first so you can do this right beside each other on the pan. Once all cookie dough is scooped, place in freezer for 2 hours. Now you can either bake them or place them in a freezer safe container for baking at a later time.
Remove cookie dough from freezer and bake in batches, 2 inches apart on parchment lined baking sheet, at 350 degrees for 15 to 17 minutes.