MIX sugar and cornstarch in medium saucepan. Gradually stir in water until blended. Bring to boil on medium heat; cook 8 min., stirring constantly. Remove from heat. Add dry gelatin mixes and zest; stir 2 min. until gelatin is completely dissolved. Refrigerate 1 hour or until slightly thickened, stirring occasionally.
MEANWHILE, microwave marshmallows and milk in medium microwaveable bowl on HIGH 1 min. or until marshmallows are puffed. Stir until marshmallows are completely melted and mixture is well blended. Stir in vanilla; cool 5 min. Blend in sour cream. Pour into 9-inch square pan sprayed with cooking spray. Refrigerate until ready to use.
SPOON gelatin mixture gently over marshmallow layer in pan. Refrigerate 2 hours or until firm. Sprinkle with graham crumbs. Unmold dessert onto serving plate just before serving.