Preheat oven to 350 degrees F. Prepare your baking sheet with a baking mat and set aside.
In a medium bowl, whisk the butter, vanilla, whole egg and egg yolk until light and fluffy. tg
Add in sugar and whisk until no lumps remain.
Mix in flour, cornstarch, baking soda and salt, just until combined. Gently fold in trail mix and incorporate evenly. The dough will be soft, thick and appear somewhat greasy.
Cover the mixing bowl with Saran Wrap. Place in the refrigerator for a 2-3 hours, or overnight if possible. Remove from the refrigerator at least 10 minutes before baking.
Preheat oven to 350 degrees F. Prepare 2 cookie sheets with a silicone baking mat and set aside.
Using an cookie scoop, scoop cookies 2-3 Tbsp in size. Best to shape the cookies taller versus wider, as it will help create thick, chewy cookies. Please cookies several inches apart on your cookie sheet.
Bake cookies 12 minutes, until edges start to brown. Remove from oven and allow to cool (cookies will continue to finish cooking once removed from the oven).
After ten minutes, transfer cookies to a wire rack to finish cooling. Cookies stay fresh covered at room temperature for up to 1 week after baking