Preheat oven to 350°F. Line 12 muffin pan holes with liners.
Stir together graham cracker crumbs, sugar, and melted butter. Press about 2-2 1/2 tablespoons of the mixture into the bottom of each muffin liner and press to compact.
Beat room temperature cream cheese with a hand mixer until fluffy. Beat in room temperature sour cream. Beat in egg, sugar, and vanilla extract. (Room temperature ingredients are a must to avoid lumps!)
Evenly divide (about 2-2 1/2 tablespoons each) cheesecake among crusts. Bake for 18-20 minutes, until they just begin to brown. Cool completely and refrigerate for at least 3 hours or overnight.
Make the ganache (about 2-3 hours before you're ready to serve): place cream and chocolate chips in a microwave safe bowl or measuring cup. Heat for 45 seconds, then whisk until completely smooth (may take more or less time depending on your microwave). Let sit until it's thick enough that you can spread it but it's not drippy.
To assemble: spread ganache over the chilled cheesecakes. Toast a marshmallow over a gas stove or BBQ and place on top (or use a kitchen torch). Serve immediately after you've placed the marshmallow on top.
Cheesecakes can be made and frozen for up to one month before topping. Once topped with ganache and marshmallow, serve immediately.