In a medium skillet, toast the pecans on medium heat for 2-3 mins until lightly browned. Set Aside.
In a bowl, combine flour, cornstarch, baking soda, and salt. Set Aside.
With a hand or stand mixer, beat butter, sugar, brown sugar for 2-3 mins until light and creamy. Reduce speed and add in the egg and vanilla. Then gradually add in the flour mixture. Beat until homogenous. Then turn the mixer off and fold in the toasted pecans. Refrigerate for 2 hour (up to overnight)
Preheat the Oven to 350 degrees. Line the cookie pans with parchment. Scoop dough into balls and place 2 inches apart. Bake for 9-11 mins or until browning on the edges. Cool on a wire rack and serve.