Put oven rack in middle position, and preheat oven to 350°F. Grease an 8-inch square baking pan with butter; line pan with parchment paper, leaving a 2-inch overhang on 2 sides. Arrange graham crackers in 1 even layer in the pan (some pieces will need to be broken or cut down to fit).

Melt butter and chocolate in a medium heat proof mixing bowl set over a saucepan of simmering water, stirring occasionally, until smooth.

Whisk together flour, baking powder, cocoa powder, and salt in a small mixing bowl.

Whisk together sugar, eggs, and vanilla in a large bowl, then pour in chocolate mixture, whisking until combined well. Whisk in flour mixture, and transfer batter to baking pan. Smooth out the batter with an offset spatula.

Bake until the top is shiny, the sides have begun to pull away slightly from the pan, and the center has puffed up but is still slightly jiggly (a wooden pick or skewer will not come out clean), about 45-50 minutes. Cool brownies completely in the pan on a wire rack.

Once the brownies are cool, preheat the oven to its low-broil setting. Arrange the marshmallows in 1 even layer on top of the brownies. Broil for 2-3 minutes, or until the marshmallows are golden brown, rotating the pan and checking on the brownies every 30 seconds. Evenly distribute about half of the chocolate bar pieces on top of the marshmallows; evenly distribute the cereal as well.

Melt the remaining chocolate and drizzle on top of the brownies.

Once the chocolate has set up, gently transfer the brownies from the pan to a cutting board using the parchment sling, and remove paper. Cut the brownies into 16 squares.