Step 1: Preheat your oven to 375°F (190°C). Line a 13x9-inch baking pan with parchment paper, ensuring the paper overhangs on the sides for easy lifting.
Step 2: Arrange the graham crackers in a single layer to cover the bottom of the prepared pan. Break them into smaller pieces as needed to fit snugly.
Step 3: In a large mixing bowl, cream together the softened unsalted butter, granulated sugar, and light brown sugar using an electric mixer until the mixture is light and fluffy.
Step 4: Beat in the egg, one at a time, until well combined. Stir in the vanilla extract, scraping down the sides of the bowl to ensure everything is fully incorporated.
Step 5: In a separate medium bowl, whisk together the all-purpose flour, baking soda, and salt. Gradually add this dry mixture to the wet ingredients, mixing on low speed until just combined and no streaks of flour remain.
Step 6: Gently fold in the semi-sweet chocolate chips and mini marshmallows into the cookie dough mixture.
Step 7: Carefully spread the cookie dough evenly over the graham cracker layer in your prepared baking pan, pressing it down lightly to form an even layer.
Step 8: Bake for 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center (avoiding marshmallows) comes out clean. The top should be lightly golden.
Step 9: Immediately upon removing the pan from the oven, arrange the broken pieces of Hershey’s Milk Chocolate Bars evenly over the hot cookie bars. Allow them to sit for a few minutes to melt, then gently spread the melted chocolate with the back of a spoon if desired.
Step 10: Let the S'mores Cookie Bars cool in the pan for at least 20-30 minutes before using the parchment paper overhang to lift them out onto a cutting board. Once slightly cooled, cut them into squares and serve warm for the best gooey experience.