Step 1: Preheat your oven to 325°F (160°C). Grab your 9-inch springform pan.
Step 2: In a medium bowl, mix graham cracker crumbs and sugar. Pour in melted butter and mix until evenly moistened.
Step 3: Pour the crumb mixture into the springform pan and press down firmly into an even layer.
Step 4: Pre-bake the crust in the preheated oven for 10 minutes. Remove and let cool completely on a wire rack.
Step 5: While crust cools, in a large bowl, beat room temperature cream cheese and sugar until smooth and creamy.
Step 6: Add eggs one at a time, mixing on low speed just until combined. Do not overmix.
Step 7: Gently stir in vanilla extract and sour cream until just incorporated.
Step 8: Gently fold in the melted and cooled semi-sweet chocolate with a spatula until no streaks remain.
Step 9: Pour the filling over the cooled crust, smooth the top, and bake at 325°F (160°C) for 45-50 minutes, until edges are set and center is slightly jiggly.
Step 10: Turn off the oven, open the door slightly, and let the cheesecake cool in the oven for about an hour.
Step 11: Remove from oven, let cool completely on a wire rack, then cover and refrigerate for at least 6 hours (preferably overnight).
Step 12: Once well-chilled, arrange mini marshmallows evenly over the top.
Step 13: Toast the marshmallows using a kitchen torch until golden brown, or place under a broiler for 1-2 minutes, watching carefully.
Step 14: Melt chopped milk chocolate and drizzle it over the toasted marshmallows.
Step 15: Unlatch springform pan, slice, and serve. Enjoy!