01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a small bowl, combine graham cracker crumbs, melted butter, and sugar. Press mixture evenly into the bottom of the prepared pan to create a uniform crust.
03 - In a medium bowl, blend melted butter with sugar. Whisk in eggs and vanilla extract until the mixture is smooth.
04 - Sift unsweetened cocoa powder, all-purpose flour, salt, and baking powder into the wet mixture. Fold gently until just incorporated.
05 - Pour the prepared brownie batter over the graham cracker base. Smooth the surface using a spatula.
06 - Bake for 28 to 32 minutes, or until a toothpick inserted in the center yields a few moist crumbs.
07 - Immediately sprinkle mini marshmallows evenly across the hot brownie. Return the pan to the oven for 3 to 4 minutes, until the marshmallows are puffed and lightly golden.
08 - Place the pan on a wire rack and allow the brownies to cool completely inside the pan.
09 - Once cooled, drizzle with melted milk chocolate over the marshmallow layer. Chill until the chocolate sets.
10 - Cut brownies into squares. Just before serving, top each portion with a scoop of vanilla ice cream and a splash of root beer or cola to create a soda float effect. Serve immediately.