Source: Joy the Baker adapted from Bon Appetit, October 1991
1 1/2 cups all purpose flour
2 teaspoons baking powder
1 teaspoon salt
1 cup (2 sticks) unsalted butter
6 ounces unsweetened chocolate, chopped
5 large eggs
1 1/4 cups dark brown sugar
1 cup granulated sugar
2 teaspoons vanilla
1 cup graham cracker, roughly crushed with your hands
12 big marshmallows
Optional: diced 4 additional marshmallows and stirred them into the batter. These marshmallows dissolve when baked, so you can leave them out, or throw them in.
Preheat oven to 350°F. Butter 9×13-inch baking pan with 2-inch-high sides. Combine first 3 ingredients in small bowl. Stir butter and chocolate in a medium sized bowl over a heavy saucepan of simmering water. Stir chocolate and butter in this double boiler until melted and smooth.
Beat eggs, sugar and vanilla in large bowl to blend. Stir in warm chocolate mixture, then dry ingredients. Fold in graham crackers. Pour batter into prepared pan. Dot with 12 large marshmallows. Bake until toothpick inserted into center comes out with moist crumbs attached, about 30- 40 minutes minutes.
Marshmallows will be browned and puffy but will deflate as the brownies cool. Cool for at least 20 minutes than slice with a sharp knife, cleaning the knife with hot water if it gets too messy and sticky. Serve or wrap individually in wax paper for storing.