Preheat oven to 325°F and line a 9x13 inch pan with parchment paper.
Combine graham cracker crumbs, melted butter, and 3 tablespoons sugar, press into pan, and bake for 8 minutes.
Beat cream cheese and remaining sugar until smooth, then add eggs and vanilla and mix until combined.
Pour cheesecake filling over the warm crust and bake for 25 minutes until just set.
Remove from oven, drizzle melted chocolate over the top, and scatter marshmallows evenly.
Broil for 1 to 2 minutes until marshmallows are golden, then cool completely and refrigerate for 2 hours before cutting.