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Special equipment: a 9-inch springform pan
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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.
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Pulse the graham crackers in a food processor until finely ground.
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Add the butter, hot chocolate mix and salt, and pulse until the mixture comes together and holds its shape when squeezed.
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Pour the mixture into the prepared pan, and press it into the bottom to make an even layer.
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Bake until the crust is just firm, about 10 minutes.
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Let cool completely.
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Place the pan on a large piece of foil, and fold the foil up the sides of the pan.
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(This helps seal the pan, to keep water out while baking.)
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For the filling: Wipe out the food processor.
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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.
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Add the eggs and vanilla, and pulse until incorporated.
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Pour the filling into the crust, and set the pan inside a roasting pan.
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Fill the roasting pan with enough hot water to go halfway up the side of the springform pan (but not above the foil).
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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.
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Turn the oven off, and open the door for a few seconds to let out some heat.
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Close the door, and let the cheesecake sit in the oven until cooked completely through, 1 hour more.
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Transfer the springform pan to a cooling rack.
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Run a knife around the edge, and let the cheesecake cool to room temperature.
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Wrap and refrigerate overnight or up to 2 days.
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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.
