Step 1: Prepare the Crust

1. Preheat your oven to 325°F (160°C). Grease a 9-inch springform pan and line the bottom with parchment paper.
2. In a medium bowl, combine the graham cracker crumbs, sugar, and melted butter. Mix until evenly moistened.
3. Press the mixture firmly into the bottom of the prepared pan to form an even crust. Use the back of a measuring cup for a smooth finish.
4. Bake the crust for 10 minutes, then set aside to cool while preparing the filling.

Step 2: Make the Cheesecake Filling

1. In a large mixing bowl, beat the cream cheese and sugar together with an electric mixer on medium speed until smooth and creamy.
2. Add the eggs one at a time, mixing on low speed after each addition until just combined.
3. Beat in the melted chocolate, sour cream, and vanilla extract until smooth and well-blended.
4. Pour the filling over the cooled crust, spreading it evenly.

Step 3: Bake the Cheesecake

1. Place the springform pan in a larger roasting pan. Fill the roasting pan with hot water until it reaches halfway up the sides of the springform pan. This water bath prevents cracking and ensures even baking.
2. Bake the cheesecake for 60–70 minutes, or until the edges are set and the center has a slight jiggle.
3. Turn off the oven and leave the cheesecake inside with the door cracked open for 1 hour to cool gradually.

Step 4: Chill the Cheesecake

1. Remove the cheesecake from the oven and the water bath. Let it cool to room temperature on a wire rack.
2. Cover the cheesecake with plastic wrap and refrigerate for at least 4 hours, preferably overnight, to set completely.

Step 5: Add the Marshmallow Topping

1. Once the cheesecake is fully chilled, preheat your broiler. Spread the mini marshmallows evenly over the top of the cheesecake.
2. Place the cheesecake under the broiler for 1–2 minutes, watching closely to prevent burning. Remove when the marshmallows are golden and toasted.
3. If desired, sprinkle graham cracker crumbs over the marshmallows and drizzle with melted chocolate for extra flair.

Step 6: Slice and Serve

1. Use a sharp knife dipped in hot water to slice the cheesecake, wiping the blade clean between cuts for neat slices.
2. Serve chilled or at room temperature, garnished with additional marshmallows or chocolate sauce if desired.