Preheat your oven to 325°F. Spray a 9-inch springform cake pan with cooking spray, and line the outside bottom with tin foil to prevent water from getting into the bottom of the pan. Set aside.

In a medium bowl, combine the graham cracker crumbs and granulated sugar.

Add the melted unsalted butter to the crumb mixture and mix until combined.

Press the mixture into the bottom of the 9-inch springform pan, creating an even layer and going halfway up the sides.

Bake the crust in the preheated oven for 14 minutes. Remove from the oven and let it cool while you prepare the cheesecake batter.

Keep your oven at 325°F. Pour water into a sheet pan or roasting pan, filling up only a little less than halfway. Set aside.

In a medium microwave-safe bowl, heat the cream until hot and bubbly (about 1 minute). Add the chocolate chips. Let it sit for 2-3 minutes, then stir. If there are still chunks of chocolate, microwave for an additional 30 seconds.

In a large mixing bowl, using a handheld electric mixer or in a stand mixer fitted with the paddle attachment, beat the room temperature cream cheese on medium speed until smooth and creamy (about 2 minutes).

While the mixer is on low, gradually add the granulated sugar to the cream cheese, continuing to beat until the mixture is well combined and smooth.

With the mixer on low speed, add the egg yolk and whole eggs all at once and mix until incorporated. Scrape down the sides and bottom of the bowl with a rubber spatula.

Gently fold in the melted chocolate/cream mixture, salt, vanilla extract, and cornstarch with a rubber spatula until smooth, making sure there are no lumps. Be careful not to overmix to avoid introducing excess air bubbles.

Pour the cheesecake batter into the prepared crust and spread it evenly with a small offset spatula or the back of a spoon.

Place the springform pan into the water bath and bake for 60-65 minutes, or until the middle of the cheesecake has a slight jiggle and the top begins to slightly brown.

Let the cheesecake cool on the counter for an hour, then refrigerate for at least 4 hours to set up.

In the bowl of a stand mixer, whisk the egg whites and salt until frothy. Keep the mixer on the lowest speed to stabilize the mixture.

While the egg whites are whipping, combine the sugar, corn syrup, water, and vanilla in a small saucepan over medium heat. Heat until the sugar has dissolved (about 3-5 minutes).

Once the sugar has melted, increase the mixer speed to medium-high, and slowly pour the hot sugar mixture down the inside rim of the mixing bowl. Pour very slowly to temper the eggs without scrambling them (about 2 minutes).

After all the sugar has been added, turn the mixer to high speed and beat for about 5 minutes, or until the mixture is very stiff and shiny.

Gently spread the marshmallow fluff on top of the chilled cheesecake.

Using a kitchen torch, carefully toast the top of the marshmallow fluff until lightly browned.