Preheat the oven to 350 degrees F.
Put the graham crackers in a large plastic bag. Using a rolling pin or skillet, crush the crackers into fine crumbs.
In a bowl, combine the graham cracker crumbs, sugar and melted butter. Press the mixture into the bottom of lightly greased (8- by 8-inch) baking dish. Bake the crust until slightly darkened and firm to touch, approximately 10 to 12 minutes. Remove the crust from the oven and reduce the temperature to 325 degrees F, while preparing the brownie batter.
Melt the butter and semisweet chocolate together in a double boiler over simmering water. Allow to cool slightly (but not harden), then add the eggs, vanilla, and sugar. Sift together the flour, baking powder and salt, and add to the chocolate mixture. Add additional chocolate chips, if desired, at the last minute, folding them in to combine.
Pour the brownie batter over the graham cracker crust and bake for 35 to 40 minutes, taking care not to overbake. A tester stuck into the brownies will not come out clean when done cooking. The idea is to create a center with a texture close to fudge. Remove the brownies from the oven and allow to cool before topping with marshmallows.
To serve, top the brownies with marshmallows and broil until the marshmallows are melted, golden and bubbling. Serve immediately and enjoy.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.