Step 1: Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides. In a medium bowl, combine graham cracker crumbs, granulated sugar, and melted butter. Mix until moistened. Press firmly and evenly into the bottom of the prepared pan. Bake for 8-10 minutes, or until lightly golden. Allow to cool slightly (about 5 minutes).
Step 2: Arrange the broken chocolate pieces evenly over the warm graham cracker crust. Return the pan to the oven for 3-5 minutes, just until the chocolate starts to look shiny and slightly soft.
Step 3: Remove the pan from the oven. Evenly sprinkle the mini marshmallows over the chocolate layer, ensuring it's completely covered.
Step 4: Adjust your oven rack to the top position, about 6-8 inches from the broiler element. Turn your oven to the broiler setting (HI or LO). Place the pan under the broiler and watch it VERY carefully. Broil for 1-3 minutes, rotating the pan as needed, until the marshmallows are puffed, golden brown, and slightly charred in spots. Remove immediately from the oven.
Step 5: Let the s'mores bars cool in the pan on a wire rack for at least 30-45 minutes before lifting them out using the parchment paper overhang. This cooling time is crucial for the chocolate to set just enough for clean cuts. Cut into squares and serve warm. Store any leftovers in an airtight container at room temperature.