For the crust:
Thoroughly grease a mini cheesecake pan. Place the graham crackers in the bowl of a food processor and pulse until finely ground. Add in the melted butter and pulse until moistened. Divide the mixture among the cavities of the cheesecake pan, about 1 heaping tablespoon in each. Firmly press into the bottom of each cavity (a shot glass or other small object makes easy work of this). Set aside.
For the cheesecake:
In the bowl of an electric mixer, beat the cream cheese and sugar on medium-high speed until creamy and well combined. Add in the vanilla and cream and continue beating until well combined and thickened. Pour in the melted chocolate. Divide the mixture among each cavity and cover with plastic wrap.
Chill until firm, at least 4 hours or overnight.
For the frosting:
Place the egg whites, sugar, and cream of tartar in the heatproof bowl of an electric mixer. Set over a saucepan of simmering water. Whisk constantly until the sugar is dissolved and the whites are warm to the touch, 3 to 4 minutes.
Transfer the bowl to an electric mixer fitted with the whisk attachment. Start beating on low speed then gradually increase to high, beating until stiff, glossy peaks form, about 5 to 7 minutes. Add in the vanilla and beat until combined. Remove to a piping bag fitted with a plain open tip, such as the Ateco #808. Pipe a dollop onto each cheesecake.
If desired, use a kitchen torch to lightly toast the marshmallow. Garnish with a piece of Hershey’s bar. Return to the fridge until ready to serve. Cheesecakes are best served the day they are made, but may be stored in an airtight container in the fridge for up to 2 days.