FOR THE CRUST:
Preheat oven to 325°F. Grease sides and bottom of one 9-inch springform pan with cooking spray and line with parchment rounds.
In a food processor, crush Oreos into fine crumbs-you should have about 3 cups.
Pour Oreo crumbs into a medium bowl and mix in melted butter using a fork until all the crumbs are coated.
Transfer mixture to the greased 9-inch springform pan and, using the flat bottom of a glass or measuring cup, press into the pan.
Bake crust in preheated oven for 10-12 minutes. Remove from the oven and reduce the temperature to 300°F.
Spread the mini marshmallows onto the crust. Set aside.
FOR THE CHOCOLATE CHEESECAKE:
Take 1 extra large piece of aluminum foil that’s bigger than your springform pan. Place springform pan in the centre and fold aluminum up to the sides to cover the outside of the pan. This ensures that no water seeps into the pan during the water bath. Alternatively, I placed my pan in a slightly larger pot to completely protect my crust from the water.
Place chopped chocolate in a large heat-safe bowl over a saucepan of simmering water. Stir constantly until the chocolate is melted and smooth. Remove bowl from heat and set aside to cool.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a handheld mixer, beat the cream cheese on low speed until smooth and creamy.
Add both sugars, cocoa and cornstarch and continue to beat on low speed until completely combined and smooth.
Scrape down the sides of the bowl and add sour cream, heavy cream and vanilla and continue beating until smooth. Add melted chocolate and mix on low speed to combine.
Add eggs and yolk one at a time, beating slowly until just combined, do not overbeat. Scrape sides and bottom of the bowl with a rubber spatula to make sure everything is fully combined. Forcefully tap bowl on the counter a few times to release any air bubbles. Pour into prepared pan, over marshmallows and place into a larger pan.
Bring a few cups of water to a boil and carefully pour into the larger pan. The water should only be a few inches up the sides of the pan. Do not go over the top.
Bake at 300°F for about 1 hour and 20 minutes to 1 hour and 30 minutes. The cheesecake is done when the edges are set and the center jiggles just slightly.
Turn off oven and leave the cheesecake to cool for 30 minutes. After 30 minutes, crack the door open and continue cooling the cheesecake for another 30 minutes. This slow cooling helps prevent cracking.
After an hour, remove cheesecake from waterbath and aluminum foil or pot. Run a knife around the edge of the pan so the cheesecake is not sticking to the sides and allow to cool completely on a rack.
Loosely cover with plastic wrap and refrigerate until firm and chilled, about 8 hours or preferably overnight. If the cheesecake begins to “sweat” pat dry with some paper towel.
