Special equipment: a 9-inch cast-iron skillet or glass pie plate
1. Preheat the oven to 175°C.
2. Stir the biscuit crumbs, melted butter and sugar together in a medium bowl until the mixture resembles wet sand. Press the crumbs into the bottom and up the sides of a 9-inch cast-iron skillet or glass pie plate.
3. Bake the crumb crust for about 10 minutes, until a shade darker. Let cool.
4. Place the chopped chocolate in a medium bowl.
5. In a small saucepan, bring the cream to a bare simmer and then pour it over the chocolate. Let stand for 2 minutes, then stir the cream and chocolate together until totally smooth. Pour the chocolate into the crust and spread evenly. Let cool a bit.
6. Preheat the grill. Position an oven rack about 4 inches from the heating element.
7. Arrange the marshmallows on top of the chocolate. Grill the pie until the marshmallows are puffed and golden brown, 2 to 3 minutes, rotating occasionally. Serve immediately.