Wash sweet potatoes and pierce each one with a knife.
Wrap each sweet potato with aluminum foil and place them on a baking sheet.
Bake sweet potatoes for 1.5-2 hours, until fork tender inside.
Let potatoes cool before peeling the skin off and mashing it. Once cooled, take off the skin and mash the sweet potatoes with a fork until smooth.
Prepare the springform pan by wrapping outside of the pan in aluminum foil. Many springform pans, especially ones that have been used quite a bit, have a chance of leaking some water inside. Wrapping outside of the springform in foil will help keep water out.
Combine graham cracker crumbs, melted butter, vanilla extract, and pumpkin spice in a mixing bowl and mix very well.
Spray inside of the springform pan with some cooking spray, making sure you get the sides too.
Spread crumb mixture in the bottom of the prepared springform, evenly pressing it over the bottom and a little up the sides.
In a bowl of an electric mixer, beat cream cheese on medium-high speed until smooth and fluffy. Scrape sides and bottom on the bowl and beat for another minute.
Pour in sugar while beating and add eggs, one at a time. Beat it until smooth. Stop to scrape bottom and sides of the bowl.
Add mashed sweet potatoes, sour cream, vanilla extract, and heavy cream and beat, still on medium-high speed, until all mixed very well.
Lower the speed and beat in cornstarch and pumpkin pie spice. Scrape sides and bottom of the bowl and mix for a few more seconds to make sure everything is evenly mixed.
Transfer batter into the springform and spread in evenly.
Preheat oven to 325°.
Place a large roasting pan into the oven and place cheesecake springform inside the roasting pan. Carefully add hot water to the roasting pan, about half way up the side of the springform.
Bake for 80-85 minutes. Turn off the oven and open the oven door half way. Let cheesecake rest for 10 minutes and take it out of the oven. Take the foil wrap off the pan.
Let it rest for 10 minutes on the counter and then gently run a butter knife between sides of the cheesecakes and the springform to carefully separate it. (You don’t actually have to take the cheesecake out of the pan until ready to serve.)
Cool cheesecake for an hour and then cover the pan with plastic wrap and refrigerate for 6 hours or overnight.
Set the top rack of the oven about 6-7 inches away from the broiler and turn on the broiler to low setting.
Spread marshmallows over the cold cheesecake and let it toast in the oven until the top is golden brown. Take it out and cool for a few minutes before cutting.