In a medium size bowl, whisk together flour, cornstarch, baking soda, and salt. Set aside.

In a large bowl using an electric mixer, cream together butter and sugars on medium speed, about 2-3 minutes. Add egg and vanilla extract; mix until combined.

Slowly add flour mixture to wet ingredients; mix just until combined. Do not overmix.

Gently fold in chopped Kit Kat candy bar pieces.

Cover bowl and refrigerate dough for 60 minutes or overnight.

Preheat oven to 350F degrees.

Scoop dough into 2 teaspoon balls, or use a #60 disher to scoop balls of dough. Place on a ungreased baking sheet, or silicone mat/parchment paper lined baking sheet. Keep dough chilled in between batches.

Bake 7-9 minutes or until barely golden brown around the edges. Do not overbake. The tops will not brown. Do not bake longer than 9 minutes. Cookies may appear to be soft and undone, but they will set as they cool.

Cool cookies on baking sheet on wire rack for 5 minutes; transfer to a wire rack to cool completely. Eat & Enjoy!