Set aside a couple tablespoons of each: broken graham crackers, broken Hershey’s bars, mini marshmallows.
Line a baking sheet with parchment paper and set aside.
In a medium bowl, whisk together the flour, cornstarch, baking soda and salt. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment (or using a hand mixer), beat on medium speed the melted butter, brown sugar and granulated sugar until light and fluffy, about four minutes. Scrape down the sides of the bowl when necessary. Add the egg, egg yolk and vanilla. Beat until combined, 20 to 30 seconds.
With the mixer on low, slowly add the flour mixture to the dough. Careful not to over beat. With a wooden spoon or rubber spatula, stir in the broken graham cracker pieces, broken chocolate pieces and mini marshmallows.
Scoop 2 tablespoons of cookie dough (I use a medium cookie scoop), shape into a ball, and place onto the prepared baking sheet. Repeat placing each ball of cookie dough about 1- to 2-inches apart. Cover baking sheet with plastic wrap and chill in the refrigerator for at least 1 hour.
Preheat oven to 350° F. Remove plastic wrap from baking sheet and bake cookies for 10 or until almost done. Remove pan from the oven, quickly press a few marshmallows, broken pieces of chocolate and graham cracker on top of each cookie. Return to oven for 2 more minutes. You can switch the oven to broil to toast the marshmallows, but keep a close eye on the cookies as they could burn quickly on broil. Cool on the pan for 5 minutes and then transfer to a wire rack to cool completely. Store in an airtight container at room temperature.
Notes
Cooking shaping: If the cookies lost their shape in the oven, use a round cookie cutter to reshape them warm out of the oven. Frame the cookie with the cookie cutter and move it in a small circle around the cookie. Store cookies in an airtight container for up to 5 days.Freeze cookie dough for up to 3 months.