Preheat oven to 375°F.
Butter the bottom of a 12×17-inch jelly roll pan, then line the bottom with parchment or waxed paper. Butter the sides of the pan.
In a medium bowl, sift together the flour, cornstarch, baking soda and salt. Set aside.
In the bowl of a standing mixer, beat the butter, sugar and brown sugar until creamy. Add eggs, one at a time, then add vanilla extract. Mix on low speed until incorporated.
With mixer on low speed, gradually add the dry ingredient mixture to the butter mixture. Stir in the chocolate chips.
Spread the chocolate chip cookie dough batter evenly in the pan, leaving about 1 inch of space between the edge of the dough and the sides of the pan on all sides.
Bake for 15 to 20 minutes. You want the cookie slightly underbaked.
Cut the cookie down the center of the longer side, to make two rectangles. If cookie is very soft, you can freeze it for 1 hour to set it before assembling the sandwich.