Step 1: Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: In a medium bowl, combine the crushed graham cracker crumbs, melted unsalted butter, and powdered sugar. Mix until the crumbs are evenly moistened.
Step 3: Press the graham cracker mixture firmly and evenly into the bottom of the prepared pan. Use the back of a spoon or a flat-bottomed glass to ensure a compact crust. Place the pan in the refrigerator to chill.
Step 4: In a medium saucepan, gently heat the heavy cream over medium heat until it just begins to simmer around the edges. Do not let it boil vigorously.
Step 5: Remove the cream from the heat and immediately pour it over the semi-sweet chocolate chips in a heatproof bowl. Let it sit undisturbed for 5 minutes.
Step 6: After 5 minutes, whisk the mixture gently from the center outwards until it is smooth, glossy, and fully combined into a luscious ganache.
Step 7: Pour the warm ganache evenly over the chilled graham cracker crust. Gently tilt the pan to ensure it spreads to the edges. Return the pan to the refrigerator and chill for at least 1 hour, or until the chocolate layer is firm.
Step 8: Once the chocolate layer is firm, remove the pan from the refrigerator. Evenly scatter the mini marshmallows over the chocolate layer, covering it completely.
Step 9: Using a kitchen torch, carefully toast the marshmallows until they are golden brown and slightly caramelized on top. Work quickly and keep the torch moving to prevent burning. (Alternatively, you can place the pan under a broiler for 1-2 minutes, watching constantly, until golden).
Step 10: Return the bars to the refrigerator for another 30 minutes to allow the marshmallows to set slightly and the bars to firm up completely.
Step 11: Once thoroughly chilled, use the parchment paper overhang to lift the entire slab of bars out of the pan. Place on a cutting board and cut into 16 squares using a sharp, warm knife. Serve immediately or store.