Preheat oven to 350°F. Spray a 9-inch deep dish pie pan with cooking spray.
Graham Cracker Crust: Combine cracker crumbs, sugar and 1/3 cup melted butter in pie pan. Mix well and press into bottom and up the sides of the pan. Bake for 10 minutes. Remove from oven.
Chocolate Filling: Bring corn syrup and 1/2 cup butter to boil in a large saucepan over medium heat, stirring frequently. Remove from heat immediately. Add chocolate and stir until melted. Beat in sugar, eggs and vanilla using using a wooden spoon or electric mixer. Stir in flour and mix until well blended.
Pour batter into crust. Bake for 27 to 35 minutes or until wooden pick inserted in center comes out nearly clean. Sprinkle with marshmallows. Return to oven and bake for 3 to 5 minutes until marshmallows have melted and are lightly toasted in places. Remove from oven and let cool 10 minutes. Serve warm OR chill for 2 hours and serve cold. For best results, cut slices with a wet knife.