Preheat oven to 325 degrees. In a small bowl, mix cracker crumbs and sugar; stir in melted butter. Press onto bottom and 1 in. up sides of a greased 9-in. springform pan. Place pan on a baking sheet. Bake 10 minutes. Cool on a wire rack.
In top of a double boiler or a metal bowl over barely simmering water, melt chocolate chips with ¾ cup milk; stir until smooth. Pour into crust; freeze just until chocolate is set, about 20 minutes.
In a large bowl, beat cream cheese until smooth. Beat in marshmallow crème, cornstarch, vanilla extract and remaining milk. Add eggs; beat on low speed just until blended. Pour over chocolate layer. Return pan to baking sheet.
Bake 65-75 minutes or until center is almost set. Sprinkle with marshmallows. Bake 6-8 minutes longer or until marshmallows are light brown. Cool on a wire rack 10 minutes. Loosen sides from pan with a knife. Cool 1 hour longer.
Refrigerate overnight, covering when completely cooled. Remove rim from pan.