Step 1

Preheat oven to 350F. Line a large half sheet baking tray with a silicone baking mat or parchment paper and set aside.

Step 2

In a large mixing bowl, use a hand mixer or stand mixer cream together butter and sugars on medium speed until light and fluffy, about 2 minutes.

Step 3

Add egg and vanilla, and beat on low until incorporated. Add flour, cornstarch, baking soda, and salt and beat until just combined.

Step 4

Add 1 cup of the graham cracker pieces, 1 cup marshmallows, and chocolate chips. Fold in until evenly incorporated in the dough.

Step 5

Use a medium cookie scoop to scoop out 2 tablespoons of cookie dough. Use your hands to roll it into a ball (this will ensure cookies with smooth edges). Place the cookie balls onto the lined baking sheet, spaced at least 2 inches apart (you can fit about 6-8 on a large cookie sheet).

Step 6

Bake for 10-12 minutes, until edges just start to brown. DO NOT OVERBAKE. The cookies may not look done in the middle, but they will continue to cook as they are cooling.

Step 7

Immediately press in the remaining marshmallows, graham crackers and Hershey bar pieces into the top of each cookie. Let it sit to start cooling, or optionally, pop into the broiler for about 1 minute to toast the marshmallows on top but keep a close eye on the cookies as they can burn fast.

Step 8

Allow the cookies to cool in the baking sheet for 5 minutes, then carefully transfer to a wire cooling rack to cool completely.