Preheat oven to 350 degrees F. In a mixing bowl, stir together graham crackers, melted butter, and salt until evenly moistened.
Firmly press mixture into the bottom and up the sides of a 9-inch (about 1 1/4-inches deep) pie plate or a 9-inch non-stick tart pan. Bake in preheated until golden brown, about 8 to 9 minutes. Remove from oven and allow to cool completely.
Make the Filling
Place chopped semi-sweet chocolate, milk chocolate and 2 Tbsp butter in a mixing bowl. Bring cream just to a very light boil in a small saucepan. Pour hot cream over chocolate mixture and stir until melted*.
Pour mixture into cooled crust and spread into an even layer. Chill in refrigerator 1 hour until set.
Make the Topping
Add egg whites, sugar and salt to a very clean heatproof bowl and set over a saucepan of barely simmering water. Heat mixture, stirring constantly, until it registers 160 degrees on a candy thermometer, about 3 - 5 minutes.
Remove bowl from saucepan, add cream of tartar and beat mixture with an electric stand mixer fitted with a whisk attachment on moderately high speed until stiff glossy peaks form, about 5 - 8 minutes. Add in vanilla just near the end.
Spread fluff mixture over set chocolate filling. Toast topping with a kitchen torch. Cut into slices when ready to serve. Store pie in refrigerator up to 3 days.
Notes
*If the chocolate doesn't melt entirely, you can put the mixture in the microwave and melt on 50% power in 15 second intervals, stirring well between until smooth.
Make sure no drop of egg yolk gets into the marshmallow meringue mixture. Even the smallest amount of egg yolk can ruin a meringue.