Special equipment: A 9-inch springform pan
For the crust: Position a rack in the center of the oven, and preheat to 325 degrees F. Butter the inside of a 9-inch springform pan.
Pulse the graham crackers in a food processor until finely ground. Add the butter, hot chocolate mix and salt, and pulse until the mixture comes together and holds its shape when squeezed. Pour the mixture into the prepared pan, and press it into the bottom to make an even layer. Bake until the crust is just firm, about 10 minutes. Let cool completely. Place the pan on a large piece of foil, and fold the foil up the sides of the pan. (This helps seal the pan, to keep water out while baking.)
For the filling: Wipe out the food processor. Pulse the cream cheese, sugar, hot chocolate mix and sour cream until completely smooth, without any lumps, scraping down the side of the bowl a few times. Add the eggs and vanilla, and pulse until incorporated. Pour the filling into the crust, and set the pan inside a roasting pan. Fill the roasting pan with enough hot water to go halfway up the side of the springform pan (but not above the foil).
Bake the cheesecake until the edge is set but the center jiggles slightly (when you gently shake the pan, the cheesecake should move in a cohesive wave motion), 1 hour 10 minutes to 1 hour 20 minutes. Turn the oven off, and open the door for a few seconds to let out some heat. Close the door, and let the cheesecake sit in the oven until cooked completely through, 1 hour more.
Transfer the springform pan to a cooling rack. Run a knife around the edge, and let the cheesecake cool to room temperature. Wrap and refrigerate overnight or up to 2 days.
Before serving, cut the marshmallows on the diagonal, and arrange them, cut-side down, in a neat line all over the top edge of the cheesecake.
Try to avoid hot chocolate or cocoa mixes that contain milk chocolate or powdered dairy ingredients.
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