Prep the pan and oven: Heat the oven to 350°F (175°C). Line an 8x8-inch or 9x9-inch baking pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment edges to prevent sticking.

Mix the crust: In a bowl, stir 2 cups graham cracker crumbs with 1/3 cup brown sugar. Add 1/2 cup melted butter and mix until the crumbs feel like wet sand and hold together when pressed.

Press and par-bake: Firmly press the crumb mixture into an even layer in the pan, packing the corners well. Bake for 8–10 minutes until set and fragrant. This step keeps the bars from crumbling later.

Add the chocolate: Sprinkle 1 1/2 to 2 cups chocolate chips evenly over the hot crust. Let sit for 2–3 minutes to soften, then gently spread into a smooth layer with an offset spatula or the back of a spoon.

Top with marshmallows: Scatter 3–4 cups mini marshmallows over the melted chocolate. Don’t press down; they’ll settle as they bake.

Bake to toast: Return the pan to the oven for 8–12 minutes, until the marshmallows are puffed and mostly golden. For deeper color, switch to broil for 30–60 seconds. Watch closely—marshmallows can burn fast.

Cool and set: Let the bars cool in the pan for at least 1 hour. For clean slices, chill 20–30 minutes after cooling to help the chocolate firm up.

Slice and serve: Lift out with the parchment overhang. Use a sharp, lightly oiled knife to cut into squares. Wipe the blade between cuts for neat edges.