For the Pie Crust: Preheat oven to 175C, and line a 7-inch springform pan with parchment paper.

In a large bowl, combine cookie crumbs and butter until moistened and mixed through. Press the crust into bottom and three-quarters of the way up the sides of the pan.

Bake crust for 10 minutes, then let cool.

For the Frozen Hot Chocolate Mousse: Combine chocolate and salt in a large bowl.

Heat 1/2 cup cream in a small saucepan over medium heat until boiling. Pour into the bowl and let stand 5 minutes to melt chocolate before mixing to combine. Stir until glossy and smooth. Let cool slightly to thicken.

In a large chilled bowl, beat the remaining 1 1/2 cups cream using a hand or stand mixer until soft peaks. Gradually add sugar and beat just until stiff peaks form.

Add in hot chocolate, and beat to mix.

Add cooled melted chocolate mixture in 3 additions, gently folding to combine between each addition.

Pour into crust, then chill, covered with plastic wrap, in freezer for at least 2 hours to set.

For the Marshmallow Frosting:
In a small saucepan set over high heat, stir together 1 1/2 cups sugar, cream of tartar and water. Bring to a boil, then turn heat to medium-low and let reduce slightly, without stirring, for 4 to 5 minutes. Set aside.

Beat egg whites in the bowl of a stand mixer fitted with the whisk attachment gradually increasing to medium high until soft peaks form.

Turn mixer to high and gradually add in remaining 1/2 cup sugar. Once combined, very carefully pour hot sugar syrup into egg whites. Beat for 5 to 10 minutes until the bowl of the mixer has cooled to the touch and the meringue is thick and glossy.

To assemble: Remove pie from freezer and remove from springform pan. Spoon the marshmallow mixture on top of the pie. Use a kitchen torch to lightly toast the marshmallow topping before serving