3 (1.55-oz.) Hersheys bars, broken into squares of 4 pieces
Preheat oven to 375º and line two large baking sheets with parchment paper. Make cookie dough: In a large bowl using an electric mixer (or in a stand mixer), beat butter and sugars until light and fluffy, about 2 minutes. Slowly beat in vanilla extract and eggs until evenly incorporated.
In a separate bowl, mix flour, baking soda, and salt. Stir into butter-sugar mixture until just combined, then fold in chocolate chips.
Make s'mores: Cut each marshmallow in half lengthwise. Sandwich halved marshmallow and one square chocolate between two squares of graham cracker. Using two large scoops (about 1/4 cup each) of cookie dough, cover the entire s'more tightly until no graham is visible. Repeat until remaining dough. Refrigerate on baking sheets for 10 to 15 minutes.
Bake cookies until lightly golden, 16 to 18 minutes. Let cool minutes on baking sheet for 10 minutes before eating (if you can stand it).