In a mixing bowl, combine graham cracker crumbs and melted butter, stirring until evenly coated and the mixture resembles wet sand.
Press the graham cracker mixture firmly into the bottom of a lined or lightly greased baking pan, creating an even crust layer.
Spread the semi-sweet chocolate chunks evenly over the crust, gently pressing them down so they nestle into the crust layer.
Bake the crust with chocolate in the oven at 350°F (175°C) for about 8-10 minutes, or until the chocolate is just starting to melt and bubble.
Remove the pan from the oven and immediately sprinkle the marshmallows evenly over the melted chocolate layer, covering all the chocolate surface.
Using a kitchen torch or placing the pan under the broiler, toast the marshmallows until golden brown and slightly smoky, about 1-2 minutes. Keep a close eye to prevent burning.
Let the bars cool completely in the pan for at least 2 hours or until the marshmallows are firm and set.
Use a sharp knife to cut the bars into squares, wiping the blade between cuts for clean edges.
Enjoy the sticky, crispy, melty layers with a perfect balance of crunch and gooeyness in every bite.
