Pie Shell: Combine the crushed biscuits and caster sugar in a medium pie dish. Add melted butter and mix it together until a base forms. Press into the pie dish.
Filling: Put the cream in a saucepan on low heat. Once it starts to simmer, remove the saucepan from heat and add in the melted chocolate. Stir with a spatula until it is combined, then whisk in the caster sugar and salt. Add the lightly beaten eggs, while whisking constantly. Pour the filling over the biscuit base.
Bake for 30 to 40 minutes, or until the chocolate filling is set. Remove from the oven and top with marshmallows (as many as possible).
Place under the grill on low for 2 to 3 minutes, until it gets golden.