1Position a rack in the middle of the oven and preheat to 350°F. Grease a 9-by-13-inch baking dish and line it with foil. Grease the foil.
2In a saucepan, warm the butter and chocolate over low heat, stirring often, just until melted. Remove from the heat and whisk in the sugars. Whisk in the eggs slowly, beating well. Whisk in the vanilla and salt.
3Sift the flour and cocoa over the chocolate mixture and, using a rubber spatula, stir in until just blended. Stir in the graham crackers and marshmallows. Pour into the prepared dish and spread evenly.
4Bake until a toothpick inserted into the center comes out almost completely clean with a few moist crumbs, about 25 minutes. Let cool for 10 minutes in the dish, then remove from the dish using the foil to lift them out; let cool completely on a wire rack.