Prepare: Preheat oven to 350°F (175°C). Line your 9x13 pan with parchment paper, leaving an overhang on the sides for easy removal.
Make Crust: Combine graham cracker crumbs, melted butter, and sugar. Press firmly into the bottom of the pan. Bake for 8 minutes.
Make Brownies: Whisk melted butter and sugar. Add eggs and vanilla. Fold in cocoa, flour, and salt until just combined.
Bake Base: Pour brownie batter over the crust. Bake for 22-25 minutes.
Add Marshmallows: Remove pan, sprinkle marshmallows (or spread fluff) evenly over the top. Return to oven for 3-5 minutes until marshmallows are puffed and toasted golden.
Cool and Serve: Let cool completely before lifting out of the pan and slicing with a clean, hot knife.