Martha transforms the nostalgic combination of chocolate, marshmallow, and graham cracker from campfire classic to the most sophisticated mini dessert (the "marshmallow" is meringue!).
Crust: Preheat oven to 350 degrees. On a parchment-lined rimmed baking sheet, arrange eight 3-by-3-inch round ring molds. In a food processor, pulse graham crackers until finely ground. Add flour, salt, butter, and sugar; pulse to combine. Transfer mixture to another rimmed baking sheet and bake 10 minutes. Using a bench scraper or metal spatula, coarsely chop until no large clumps remain. Bake 10 minutes more. Remove and chop again; set aside to cool.
Meanwhile, melt milk chocolate in a small heatproof bowl set over (not in) a pot of simmering water. Transfer graham cracker mixture to a large bowl. Add melted milk chocolate and stir to combine. Add 1/4 cup to each ring mold and press with a small spoon to pack tightly. Refrigerate until set.
Mousse Filling: In a small bowl, sprinkle gelatin over 1 tablespoon cold water; let stand until softened, about 5 minutes. Melt semisweet chocolate and butter in a heatproof bowl set over (not in) a pot of simmering water, stirring occasionally, until smooth. Add gelatin mixture, stirring until dissolved. Remove from heat; add egg yolks and whisk until well combined. Let cool to room temperature.
In a large bowl, whisk together egg whites and salt until stiff peaks form. Whisk one-third of egg-white mixture into semisweet-chocolate mixture; gently fold in remaining egg-white mixture. Gently fold in whipped cream. Divide mousse evenly among ring molds (about 1/4 cup each). Refrigerate until set, at least 2 hours.
Meringue Topping: Combine egg whites, sugar, and salt in the heatproof bowl of a mixer set over (not in) a pot of simmering water. Whisk constantly until sugar is dissolved, and mixture is warm to the touch and feels completely smooth when rubbed between fingertips. Transfer bowl to mixer fitted with the whisk attachment. Starting on low speed and gradually increasing to medium-high, whisk until stiff (but not dry) peaks form. Add cornstarch and vanilla. Continue whisking until meringue is fluffy and glossy, and completely cool (test by touching the bottom of the bowl), 6 to 8 minutes.
Transfer meringue to a pastry bag fitted with a 1/2-inch round tip; pipe on top of mousse to fill mold. Using a small offset spatula, smooth meringue to make it flush with top of mold.
Refrigerate until set, about 1 hour. Let stand at room temperature 5 minutes. To unmold, place ring mold on top of a narrow glass or spice jar; gently pull mold down. If necessary, briefly torch sides of molds with a kitchen torch to loosen (be careful not to overheat). Transfer to a heatproof plate. Repeat process with remaining molds. Use kitchen torch to brown tops of meringues. Serve immediately.