Bake chocolate cake according to package directions in two 9-inch round pans and let cool completely.
Mix graham cracker crumbs, melted butter, and 2 tablespoons sugar to form a crumble and set aside.
Place egg whites, granulated sugar, and cream of tartar in a heatproof bowl over simmering water, whisking constantly until sugar dissolves and mixture reaches 160°F.
Remove from heat, beat with a hand mixer on high for 5 to 7 minutes until stiff glossy peaks form, then beat in vanilla.
Place one cake layer on a plate, spread a thick layer of marshmallow frosting, and press graham cracker crumble into the frosting.
Add the second cake layer; frost the entire outside with remaining marshmallow frosting.
Use a kitchen torch to toast the outside of the frosting, then drizzle with melted chocolate before serving.