Preheat the oven to 350 degrees F. Line an 8 inch square metal pan with nonstick foil.
Crust: Mix the graham cracker crumbs with the sugar, melted butter and salt (if using). Press into bottom of pan and bake for 6 minutes.
Brownies: Melt the chocolate and butter together in the microwave, a double boiler or heavy saucepan set over low heat. Let cool for about 5 minutes.
Whisk the eggs and sugar and vanilla together in a mixing bowl.
Stir the slightly cooled chocolate mixture into the egg mixture, then stir in the salt and baking powder. Add flour and stir until well blended.
Pour the brownie mixture over the graham crust, then return to oven and bake for 22-26 minutes or until set.
Remove from oven and sprinkle marshmallows over top in one generous layer.
Adjust oven rack so that brownies are about 6 inches from broiler element, then set brownies under broiler (set on high) for 1 minute or until golden. Keep a close eye, because marshmallows brown quickly!
Let cool on a rack. When cool, chill. Lift from pan and cut into squares. For a cleaner cut, freeze before slicing.