Preheat oven to 325° F. Grease a 9-inch springform pan with butter and set aside.
Use a food processor to crush graham crackers until fine crumbs form. Add sugar and melted butter. Pulse a few times until combined. Transfer mixture to prepared pan and firmly press the mixture to the bottom of the pan. I like to use the bottom of a measuring cup to press the mixture down.
Bake crust for 10 minutes. Remove from oven and set aside to cool. Keep oven on. Place springform pan on top of aluminum foil and wrap the outside of the pan tightly with the foil. Make sure the foil comes almost all the way up the sides of the pan. Prepare cheesecake mixture.
Cheesecake
Grease a baking sheet with butter. Arrange marshmallows on top. Bake marshmallows for 5 minutes until golden brown. Allow to cool slightly. Start the cheesecake filling while the marshmallows are baking.
In the bowl of a stand mixer, beat cream cheese and sugar on medium-high speed until smooth, about 4 minutes. Scrape down the sides and bottom of the bowl as necessary.
Add the sour cream, vanilla and lemon juice. Beat until combined, about 1 minute. Scrape down the sides and bottom of the bowl. Add the eggs and egg yolk, one at a time, beating just until combined after each addition. Avoid over-mixing.
Mix in the toasted marshmallows. Scrape beaters/paddle attachment as needed. It’s okay if there are a few toasty lumps in the batter. Transfer batter to springform pan with graham cracker crust.
Prepare water bath. Fill a roasting pan with hot tap water until 1 inch high. Place roasting pan in the oven and then place the cheesecake in the water-filled roasting pan.
Bake at 325° F for 60 minutes. The cheesecake is done when the pan is gently shaken and the center of the cake jiggles slightly, but the edges are set. Turn the oven off, prop the oven door open and keep the cheesecake in the water bath for 1 hour. Remove cake from water bath and transfer to the refrigerator to cool for 4 hours or overnight.
Chocolate Ganache
Place chocolate chips in a bowl and set aside.
Add heavy cream to a saucepan. Heat until scalding hot and the cream at the edge of the pan is simmering. Pour hot heavy cream over chocolate chips and let the mixture sit for 5 minutes. Stir until smooth and chocolate is melted.
Pour chocolate ganache over chilled cheesecake and spread with an off-set spatula.
Garnish cheesecake with mini marshmallows and graham cracker pieces. Slice and serve.
Notes
Block-style cream cheese is a must for cheesecake. It works significantly better than cream cheese in the tub or low-fat varieties.Store leftover cheesecake covered or in an airtight container in the refrigerator for up to 5 days.Freeze for up to 1 month. Wrap the cheesecake tightly in plastic wrap, then add a layer of foil to help prevent freezer burn.