Make the marshmallow fluff. Fill a small saucepan with about 1 inch (2.5 cm) of water and bring to a gentle simmer.
In the bowl of a stand mixer or another heatproof bowl, whisk together the egg whites, sugar, cream of tartar, corn syrup, and salt.
Set the bowl over the simmering water, making sure the bottom of the bowl does not touch the water. Whisk constantly for 4 to 5 minutes, or until the mixture reaches 160°F (71°C) and the sugar has completely dissolved.
Transfer the bowl to a stand mixer fitted with the whisk attachment. Beat on high speed for 6 to 8 minutes, or until the mixture is thick, glossy, and marshmallow-like. Beat in the vanilla.
Transfer the marshmallow fluff to a piping bag fitted with a large round tip or to a large zip-top bag with one corner snipped.
Make the hot cocoa. In a medium saucepan, whisk together the milk, cocoa powder, and sugar over medium-low heat until smooth and steaming.
Add the chopped chocolate and vanilla. Whisk until the chocolate is completely melted and the cocoa is smooth. Remove from the heat.
Decorate the cups. Pipe a generous ring of marshmallow fluff around the rim of each empty cup or mug. For ceramic mugs, pipe some of the fluff just over the outside edge so it has more surface area to grip. Sturdy paper cups naturally hold the fluff more easily.
Using a kitchen torch, carefully toast the marshmallow fluff until golden brown.
Slowly pour the hot cocoa into each cup, taking care not to disturb the toasted marshmallow rim.
Finish with chopped chocolate or chocolate shavings and serve immediately.
Styling Notes
Pipe the marshmallow fluff generously so it creates a dramatic cloud around the rim, then toast until deeply golden in spots.
Serve the cups together on a platter and let a little of the marshmallow fluff drip naturally down the sides for an extra-cozy look.
Get Creative
Use a mix of milk and dark chocolate for a richer cocoa.
Add a splash of peppermint extract for a holiday version.
Finish with crushed candy canes, chocolate curls, graham cracker crumbs, or a pinch of flaky sea salt.