Heat the oven to 350°F (175°C). Grease a metal pan suitable for broiling. Avoid exposed parchment because it can scorch under the broiler.
Combine graham crumbs and melted crust butter. Press evenly across the base and bake for 8 minutes, then set aside.
Melt brownie butter and chocolate gently. Cool for 5 minutes, then whisk in sugar, eggs, and vanilla until glossy.
Fold in flour, cocoa, and salt just until combined. Spread the brownie batter over the warm crust without dragging crumbs into it.
Bake for 23–28 minutes, until the brownie edges set and a tester has moist crumbs but no raw batter.
Scatter marshmallows evenly over the surface. Switch to the broiler and position the pan about 8 inches below the heat.
Broil for 30–90 seconds, watching continuously, until the marshmallows are lightly toasted. Remove immediately; browning can become burning very quickly.
Cool completely in the pan before slicing. Use a lightly oiled knife, wiping it frequently to cut through the sticky marshmallow topping.
