Prep the pan: Line your pan with parchment, leaving overhang for easy lifting. Preheat oven to 325°F (163°C).
Make the crust: Stir graham crumbs, sugar, melted butter, and a pinch of salt until the mixture looks like wet sand.
Press firmly into an even layer in the pan. Bake for 8–10 minutes until set. Cool slightly.
Beat the cheesecake: In a bowl, beat cream cheese and sugar until smooth and fluffy, about 2 minutes.
Mix in sour cream, vanilla, cornstarch, and salt. Add eggs one at a time, mixing on low just until combined. Do not overmix.
Bake the cheesecake layer: Pour filling over the crust.
Smooth the top. Bake 22–28 minutes, until edges are set and the center still has a slight jiggle.
Cool, then add topping: Let the pan cool on a rack for 15 minutes. Sprinkle chocolate and mini marshmallows evenly over the warm cheesecake.
Toast the topping: Return to the oven for 3–5 minutes, just until marshmallows puff and turn golden.
For deeper color, broil 20–60 seconds, watching closely.
Chill to set: Cool to room temperature, then refrigerate at least 3 hours (overnight is best). Lift out with parchment and cut into bars with a sharp knife, wiping between cuts.