4. Sprinkle the gelatin over the 1/4 cup (60ml) cold water in a small bowl. Let stand 5 minutes.
5. 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.
6. 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.)
7. 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.)
8. 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.
9. 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.