If desired, lightly toast pecans in the oven or on the stovetop.
In a bowl, whisk together flour, cornstarch, salt, and baking soda.
In a large bowl using a hand held mixer, or a standing mixer using a beater attachment, cream butter, brown sugar, and granulated sugar until combined and fluffy, about 2-3 minutes. Scrape down the sides of the bowl with a spatula as needed.
Beat in egg and vanilla until combined.
Add flour mixture and mix until combined, scraping down the sides of the bowl with a spatula as needed. Do not overmix the cookie dough once the flour has been added.
Gently fold in chocolate chips and pecans.
Spoon or drop about two tablespoon balls of the cookie dough closely together onto a baking sheet lined with parchment paper or a silicone baking mat. Add additional pecans or chocolate chips to the top of the cookies if desired.
Chill the cookies in the refrigerator for one hour.
Preheat the oven to 350 ℉.
Line another baking sheet with parchment paper or a silicone baking mat.
Put the chilled cookie dough balls onto a baking sheet, 2-3 inches apart.
Bake for 10-12 minutes until the edges start to brown. Remove from oven and let cool on the baking sheet 5-10 minutes. Once they’ve started to firm up move the cookies to cooling rack to continue cooling.
When cool, enjoy!