In a 300F degree oven, toast the chopped pecans on a baking sheet for 10 minutes.
In a large bowl, using a electric mixer to beat the butter for 1 minute on medium speed until completely smooth and creamy. On medium speed, add and beat brown sugar. On high speed, beat in eggs and vanilla. On low speed, add flour, cornstarch and baking soda until combined. Add the toasted chopped pecans, mix on low for about 5-10 seconds until evenly disbursed. Cover dough tightly with plastic wrap and chill for at least 3 hours.
Remove cookie dough from the refrigerator and allow to sit at room temperature for 20 minutes. This makes the cookie dough easier to scoop and roll.
Preheat oven to 350F. Scoop and roll balls of dough into balls and flatten onto baking sheets.
Bake the cookies for 10 minutes, until slightly golden brown around the edges. The baked cookies will look extremely soft in the centers when you remove them from the oven. Allow to cool for 5 minutes on the cookie sheet. Transfer to cooling rack to cool completely.