In a large bowl cream together the butter and shortening until well blended. Add sugar and brown sugar. Cream on medium speed for 2-3 minutes, scraping down the sides of the bowl as necessary.
Add eggs, one at a time, beating well after each addition. Add vanilla and blend well.
In a medium bowl combine the all purpose flour, cornstarch, baking soda, cinnamon and salt.
Stir flour mixture into wet ingredients by hand in 2 parts. Add quick oats and stir until everything is thoroughly combined. Cover dough with plastic wrap and refrigerate for at least 1 hour.
Preheat oven to 350℉/177℃. Line 3 baking sheets with parchment paper. Roll walnut size balls of the chilled dough and place on baking sheets, 12 on each, spacing them about 1 inch apart. Do not flatten. Bake for 14-16 minutes or until the cookies have spread out and are lightly golden on the bottom. Cool for 3-5 minutes on the baking sheets before transferring to rack to cool.