Heat oven to 350°F. Line a 9×13-inch pan with parchment and lightly grease.
Combine graham cracker crumbs with 0.33 cup melted butter. Mix until evenly moistened.
Press the mixture firmly into the pan. Bake 8 minutes, then set aside.
Whisk 0.5 cup melted butter with granulated and brown sugars until glossy and combined.
Beat in the eggs one at a time, then stir in vanilla until smooth.
Whisk flour, cocoa, baking powder, and salt in a bowl. Fold into the wet mixture just to combine.
Stir in chocolate chips. Spread batter evenly over the warm crust.
Bake 22–26 minutes, until the center is just set and a few moist crumbs cling to a tester.
Top with mini marshmallows, chopped milk chocolate, and broken grahams. Return to oven 3 minutes to toast and melt.
Cool completely in the pan. Finish with flaky salt if using, then lift out and cut into squares.