• Preheat oven to 350º.

Crust

• Mix together melted butter, graham cracker crumbs, and sugar.
• Pat into a greased, 9-inch pie plate.
• Bake for 12 minutes or till lightly browned. Cool completely.
• When the crust is cool, preheat oven to 350° again.

Chocolate Filling

• Put chocolate in a large bowl.
• Bring cream just to a boil in a heavy saucepan (or microwave in a large Pyrex measuring cup till cream starts to boil), then pour hot cream over chocolate.
• Let stand a minute or two, then whisk until chocolate is melted and the mixture is smooth.
• Whisk in egg and a pinch of salt until combined and pour into prepared crust.
• Cover the edge of pie with foil and bake until filling is almost set, about 25 minutes. Remove to rack and cool to room temperature, about 1 hour.

Marshmallow topping

• Sprinkle gelatin over 1/4 cup cold water in the metal bowl of your stand mixer (or another large heatproof bowl if using a hand mixer)and let stand until softened, about 1 minute.
• Stir together sugar, corn syrup, a pinch of salt, and remaining 1/4 cup water in a heavy saucepan. Bring to a boil over medium heat, stirring until sugar is dissolved. Boil until liquid reaches 260º.
• Begin beating water and gelatin mixture medium speed, then carefully and slowly pour in hot syrup avoiding the sides of the bowl and beaters. When all of the syrup is added, increase speed to high and continue beating for about 5 minutes. Add vanilla and beat until combined, then spoon onto center of the pie, spreading to edges with an offset spatula if needed. Chill, uncovered, 1 hour, then cover loosely with plastic wrap sprayed with non-stick cooking spray and chill 3 hours.
• To serve, brown top using a hand held kitchen torch or place pie on baking sheet, cover sides with foil and broil (about 4 inches from element) for about 3 minutes or till top is golden brown. Cool about 10 minutes before cutting.
• For easiest slicing, dip a large knife into hot water, dry knife and slice.Repeat.