If you're not making the s'mores right away, just pipe the marshmallow directly onto powdered sugar covered cookie sheets and let set until ready to use, at least 1 hour or overnight. If you're serving them right away, lay half of the graham crackers on a cookie sheet. Top with chocolate pieces to cover, then pipe "kisses" of marshmallow to cover the chocolate. (If using store-bought marshmallows, place 3 marshmallows on top of the chocolate layer. Leftover homemade marshmallows should be stored in airtight containers.) Heat the oven to 400 degrees F. Bake until the marshmallow is puffed and golden brown, about 3 to 5 minutes. Remove from the oven and top with the remaining graham crackers, pressing down slightly to make a sandwich. Serve immediately, while still warm.
Combine the water, the corn syrup, and the sugar in a saucepan fitted with a candy thermometer. Bring to a boil and boil to "soft-ball" stage, or about 235 degrees F.
Meanwhile, whip the egg whites until soft peaks form. Sprinkle the gelatin over the 2 tablespoons cold water and let dissolve. When the syrup reaches 235 degrees F, remove it from the heat, add the gelatin, and mix. Pour the syrup into the whipped egg whites. Add the vanilla and continue whipping until stiff and mostly cooled. Transfer to a pastry bag with a large plain tip.
Homemade Graham Crackers:
In a food processor or the bowl of an electric mixer, mix together the flours, sugar, baking powder, baking soda, salt, and cinnamon. Add the cold butter and mix or process until the mixture resembles coarse meal. Add the honey, molasses, water, and vanilla. Mix until the dough comes together in a ball.
Between 2 sheets of waxed paper or plastic wrap, roll the dough 1/2-inch thick. Chill for 1 hour, until firm. Set a rack in the middle of the oven and preheat to 350 degrees F. Lightly flour the dough and roll 1/8-inch thick. With a sharp knife or cookie cutter, cut into 2-inch squares. Arrange the crackers on nonstick or parchment lined cookie sheets. With a fork, prick several holes in each cracker. Bake for 15 minutes, until lightly browned at the edges. Remove from the oven and let cool in the pan.
Recipe courtesy of Gale Gand's Just A Bite by Gale Gand and Julia Moskin: Clarkson N. Potter Publishers, 2001