Prepare Baking Sheet: Line a baking sheet with parchment paper or a silicone mat.
Mix the Dough: In a large mixing bowl, stir together the 1 15.25-ounce Devil’s Food cake mix, 1/2 cup vegetable oil, and 2 large eggs until fully combined.
Add Mix-Ins: Fold in 2 cups semi-sweet chocolate chips and 1/2 cup coarsely crumbled graham crackers.
Chill the Dough: Place the dough in the refrigerator to chill for 30 minutes. While it chills, preheat the oven to 350°F.
Shape the Cookies: Use a 2-tablespoon cookie scoop to portion the dough. Flatten each portion into a disk on the prepared baking sheet (6 per batch).
Add Marshmallows: Place one marshmallow half in the center of each dough disk. Wrap the dough around the marshmallow, leaving the top slightly exposed.
Bake: Bake for 8–10 minutes, or until the edges are set and the tops look glossy. Watch closely! The marshmallow should puff but not dissolve completely.
Top and Cool: Remove from the oven. Immediately sprinkle each cookie with extra graham cracker crumbs and top with one Hershey's chocolate rectangle.
Cool Gently: Let cookies rest on the baking sheet for 5 minutes, then carefully transfer to a wire rack to cool completely.
Notes
Make-Ahead Tips: Dough can be made and chilled up to 24 hours ahead. Scoop and wrap the cookies right before baking.Freezing Tips: Freeze unbaked cookie dough balls (with marshmallows wrapped inside) on a sheet pan until solid, then store in a freezer bag for up to 2 months. Bake from frozen and add 1–2 minutes to the baking time.Reheating Tips: If you want that gooey center again, microwave cooled cookies for about 8 seconds before serving.