01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang to facilitate removal.
02 - In a small bowl, combine graham cracker crumbs, melted butter, and sugar. Press mixture evenly in the bottom of the prepared pan to create a firm crust.
03 - In a medium bowl, whisk together melted butter and granulated sugar. Incorporate eggs and vanilla extract until emulsified.
04 - Sift in cocoa powder, flour, salt, and baking powder. Stir until just blended, avoiding overmixing.
05 - Spread brownie batter evenly over graham cracker base. Smooth surface and bake for 28 to 32 minutes; a toothpick inserted should emerge with a few moist crumbs.
06 - Remove pan from oven and immediately scatter mini marshmallows over the brownies. Return pan to oven for 3 to 4 minutes, until marshmallows are puffed and lightly golden.
07 - Transfer pan to a rack and cool brownies completely within the pan.
08 - Once cooled, drizzle melted milk chocolate evenly over the marshmallow layer. Chill briefly until chocolate sets.
09 - Just before serving, cut brownies into 12 squares. Top each square with a scoop of vanilla ice cream and finish with a splash of root beer or cola. Serve immediately.