Combine the butter, sugars, egg, and vanilla in a bowl of a stand mixer. Beat using the paddle attachment for 5 minutes or until light and fluffy. (If using a hand mixer, mix for at least 7 minutes.) Scrape down the sides of the bowl, and add the cornstarch, baking soda, and salt and mix until combined. Then add the flour and mix until incorporated.

Add the ¾ cup of Snickers and chocolate chips and beat quickly, only until incorporated or you can fold in by hand.

Place balls of dough in mounds on a large plate or cookie sheet lined with a Silpat or parchment paper. (It just depends how much room you have in your fridge since you will be chilling the dough.) Add the ¼ cup of Snickers on top of the cookies. Cover loosely with plastic wrap and refrigerate for at least 2 hours or up to a few days. If you are in a hurry you can chill the dough quickly by placing it in the freezer for about 15 minutes.

Preheat oven to 350F. Line 2 cookies sheets with a Silpat or parchment paper and place cookie dough mounds on baking sheet at least 2 inches apart. Bake for 8-10 minutes, or until edges have set and tops are just beginning to set. Do not bake longer than 10 minutes, the cookies will firm up as they cool. Allow cookies to cool for 5-10 minutes before moving to a cooling rack.

Store cookies in an airtight container for up to 1 week or in the freezer for up to 3 months.