S'mores Ice Cream Pie with Salted Butter Chocolate Sauce

To make the crust

1. Mix together the Graham crackers, sugar and butter until the crumbs are thoroughly moistened. Press them into a 9-inch (23cm) pie tin or ovenproof pie plate (see Note at end of recipe), using a drinking glass to get the bottom of the pie crust flat, pressing the crumbs up the side with your fingers. Freeze the crust for 30 minutes.
2. Preheat the oven to 375ºF (190ºC). Remove the pie crust from the freezer and bake until the crust is browned, about 10 minutes. Cool on a wire rack.
3. When the crust is cool, let the ice cream soften a bit so it's spreadable, and carefully spread it into the prepared pie shell, being careful not to break the sides. (Tip: Sometimes it helps to beat the ice cream on medium speed in a stand mixer, fitted with the beater attachment, until it's smooth and soft enough to spread.) Return the pie to the freezer.

To make the marshmallow topping

1. Sprinkle the gelatin over the 1/4 cup (60ml) cold water in a small bowl. Let stand 5 minutes.
2. In a small saucepan fitted with a candy thermometer, heat the remaining 1/3 cup (80ml) cold water, corn syrup, and sugar over medium heat. Put the egg whites in the bowl of a stand mixer fitted with the whip attachment.
3. When the syrup reaches about 210ºF (99ºC), begin whipping the egg whites. When the syrup reaches 245ºF (119ºC), with the mixer on high speed, in a slow, steady stream, pour the hot syrup into the egg whites, aiming between the whip and the side of the bowl. (If the syrup hits the whip, it'll cling to the side of the bowl, rather than incorporated into the egg whites.)
4. Scrape the gelatin into the still-warm saucepan, stirring it until the gelatin is dissolved, then whip it into the egg whites. Continue to beat on high speed until the marshmallow topping is room temperature. Remove the ice cream pie from the freezer and spread the topping over the pie. (If you want to finish it later, you can return the marshmallow-topped pie to the freezer. Or you can broil the topping now and return it to the freezer as well, to serve later.)
5. To finish the pie, turn on the broiler of your oven. Place the pie on an upper rack of the oven and until the top is browned, watching it carefully so it browns evenly and doesn't burn.
6. Remove from the oven. To slice and serve, dip in a long, sharp knife in hot water and cut a slice of the pie, without wiping the blade: the hot water will prevent the marshmallow topping from sticking to the blade. For clean slices, you may need to rinse and wipe the blade of the knife clean between slices.
7. Serve with warm salted butter chocolate sauce.