Combine egg whites, sugar, and vanilla extract in the metal mixing bowl of your stand mixer and set over a pot filled with boiling water on a range, creating a double boiler.
Whisk by hand for a few minutes until the mixture reaches 110 F on a candy thermometer.
Place the bowl on a stand mixer fitted with the whisk attachment. Whisk on medium speed for 10 minutes.
Increase to high speed and whisk until stiff glossy peaks form, about 5 to 10 minutes more.
Spread a thin layer of marshmallow frosting over each cupcake.
Dip each cupcake into crushed graham crackers.
Use a piping bag fitted with a large round tip to pipe a generous swirl of frosting on each cupcake, leaving a small edge of graham cracker crumbs showing.
Using a culinary torch, toast the marshmallow frosting.
Top each cupcake with a graham cracker square, a piece of chocolate, mini marshmallows, and chocolate syrup.
Serve within 24 hours (otherwise the graham cracker will get soggy).
Video
Notes
When making the marshmallow frosting:
Make sure the inside of the bowl is totally dry before beginning.
Make sure there are no egg yolks mixed in with the egg whites.
Keep on whisking – don’t stop early.
If the graham cracker doesn't stay upright on top of the cupcakes, you can place a small toothpick behind it to support it.