Prep the Pan: Preheat your oven to 375°F (190°C). Line a 13×9-inch baking pan with parchment paper, leaving a 2-inch overhang for easy removal.
Layer Graham Crackers: Arrange the graham crackers in a single layer on the bottom of the prepared pan. Break them as needed to fit snugly.
Cream Butter and Sugars: In a large bowl, use an electric mixer to cream the butter, granulated sugar, and brown sugar until light and fluffy.
Add Wet Ingredients: Beat in the eggs one at a time, followed by the vanilla extract. Mix until smooth.
Combine Dry Ingredients: In a separate bowl, whisk together the flour, salt, and baking soda. Gradually add this to the wet ingredients, mixing just until incorporated.
Fold in Mix-Ins: Gently fold in the semi-sweet chocolate chips and 1 cup of mini marshmallows.
Assemble the Bars: Spread the cookie dough evenly over the graham crackers in the pan. Press gently to create an even layer. Sprinkle additional mini marshmallows on top if desired.
Bake: Bake for 25-30 minutes, or until the top is golden and set. If the marshmallows start to brown too quickly, loosely cover the pan with foil during the last 10 minutes of baking.
Add Chocolate Topping: Remove from the oven and immediately place the Hershey’s chocolate bar pieces on top. Allow them to soften, then spread gently with a spatula if desired.
Cool and Slice: Let the bars cool in the pan for 20-30 minutes. Use the parchment overhang to lift the bars out. Slice into squares or rectangles and serve.