Preheat oven to 325°F (160°C). In a medium bowl, combine the graham cracker crumbs and granulated sugar. Pour in the melted butter and mix until well combined. Press the mixture firmly and evenly into the bottom and slightly up the sides of a 9-inch springform pan. Bake for 8-10 minutes, or until lightly golden. Let cool completely on a wire rack.

While the crust cools, in a microwave-safe bowl, combine the chocolate chips and 2 tablespoons of butter. Microwave on high in 30-second intervals, stirring until completely melted and smooth. Pour the melted chocolate mixture over the cooled crust and spread evenly. Place the pan in the refrigerator or freezer for 10-15 minutes to set the chocolate layer.

In a very large bowl, using an electric mixer, beat the softened cream cheese on medium speed until completely smooth and lump-free (about 2-3 minutes). This is crucial for a smooth cheesecake. Add the granulated sugar, vanilla extract, and salt. Beat on medium-low speed until just combined. Do not overmix. Add the eggs one at a time, beating on low speed until just incorporated after each addition. Scrape down the sides of the bowl as needed. Do not overbeat once eggs are added, as this can incorporate too much air and cause cracks. Gently fold in the sour cream until just combined.

Water Bath (Recommended for Crack-Free Cheesecake): Wrap the outside of the springform pan tightly with several layers of heavy-duty aluminum foil to prevent water from seeping in. Pour the cheesecake filling over the set chocolate layer in the springform pan. Place the foil-wrapped springform pan into a larger roasting pan. Carefully pour hot water into the roasting pan until it comes halfway up the sides of the springform pan. Bake for 60-75 minutes, or until the edges are set but the center (about a 2-inch circle) still jiggles slightly when gently shaken. Turn off the oven, crack the oven door, and let the cheesecake cool in the water bath for 1 hour. This helps prevent cracking. Carefully remove the cheesecake from the water bath, remove the foil, and transfer it to a wire rack to cool completely at room temperature. Once cool, cover loosely with plastic wrap and refrigerate for at least 6-8 hours, or preferably overnight, to allow it to set completely.

In a medium saucepan, heat the heavy cream over medium heat until it just begins to simmer (do not boil). Remove from heat and pour the hot cream over the finely chopped chocolate in a heatproof bowl. Let sit for 5 minutes, then whisk until smooth and glossy. Let the ganache cool slightly at room temperature until it’s thick but still pourable.

Once the cheesecake is fully set, carefully remove it from the springform pan. Transfer the cheesecake to your serving platter. Pour the slightly cooled chocolate ganache over the top of the cheesecake, spreading it to cover the surface and allowing it to drip down the sides of the cake. Immediately arrange the mini marshmallows and chopped chocolate chunks on top of the ganache while it’s still wet. To Toast Marshmallows: Use a kitchen torch to carefully toast the marshmallows until they are golden brown and slightly charred in spots, as seen in the picture. If you don’t have a torch, you can place the cheesecake under a preheated broiler for 1-2 minutes, watching very closely as they can burn quickly. Optional: Sprinkle a few extra graham cracker crumbs or crushed chocolate cookies, or drizzle with more ganache or caramel. Slice with a sharp knife (dipped in hot water and wiped clean between slices for cleaner cuts) and serve immediately for the best toasted marshmallow experience.