In a medium bowl, stir together all crust ingredients, melted butter, graham cracker crumbs, sugar, and salt. Press moistened crumbs evenly on bottom and up the sides of the prepared pie plate.
Bake until crisp, 12 to 15 minutes. Cool on a wire rack to room temperature, about 45 minutes.
For the Filling:
Heat the oven to 350° F.
Place chopped chocolate in a large bowl. In a small saucepan, bring cream just to a boil. Pour hot cream over chocolate in the large bowl, and let stand 1 minute. Gently whisk until chocolate is melted and mixture is smooth.
Gently whisk egg and a pinch of salt into chocolate mixture, until just combined.
Pour chocolate mixture into the cooled crust (crust will be about half full).
Cover the edge of the pie crust with a pie shield or foil, and bake until the filling is softly set and jiggles slightly in the center when gently shaken, about 20-25 minutes.
Cool pie to room temperature on a cooling rack, filling will continue to cook and firm as it cools, about 1 hour.
Make Marshmallow topping:
In the bowl of a stand mixer, sprinkle gelatin over ¼ cup cold water, and let stand while you make the sugar syrup.
In a small 1 to 1 ½-quart saucepan, stir together sugar, corn syrup, a pinch of salt, and remaining ¼ cup water. Bring to a boil over medium-high heat, stirring until sugar is dissolved, then continue to boil over moderate heat until candy thermometer reaches 260° F (hard ball stage), about 6 minutes.
Begin beating the water and gelatin in the stand mixer at medium speed. With the beater running, carefully pour in hot syrup in a slow stream avoiding the beater and sides of the bowl. When all of the syrup is added, increase the speed to high, and continue to beat the mixture until very thick and tripled in volume, about 5 minutes. Add the vanilla and beat until combined.
Immediately spoon the topping onto the center of the pie, spreading it to cover the filling. Chill, until set, about 4 hours.
If you would like to cover the pie with plastic wrap, be sure to lightly spray the plastic wrap with non-stick spray or oil.
Before serving, brown the marshmallow topping with a food safe torch. Alternatively, you can brown the topping under a broiler. Preheat the broiler. Transfer the pie to a baking sheet, and cover the edges of the pie with a pie shield or foil. Broil 3 to 4 inches from the broiler, rotating the pie if necessary, until the topping is golden brown, about 3 minutes.
Cool the pie on a cooling rack 10 minutes.
Slice pie with a large heavy knife that’s been sprayed or brushed with oil or non-stick spray.