Prepare the pie crust first. If you are using my flaky pie crust recipe or a store-bought pie crust, fully bake it according to the recipe or package instructions and set aside to cool. If you are using a no-bake pie crust with graham crackers or Oreo crumbs, there is no need to bake it, just prepare and set aside.
Finely chop the chocolate and put it into a large bowl.
Heat the heavy cream and vanilla in a saucepan over medium heat until it is scalding hot. (This means that there is steam rising from it and you can see tiny bubbles appearing on the surface). Do not let it boil.
Pour the hot cream over the chopped chocolate and let it sit for one minute, then stir until all the chocolate has melted and you have a smooth, glossy ganache. Put roughly a third of the ganache (about ¾ cup) into a separate bowl. We will use this as a topping later. Set both bowls aside while you make the meringue.
Make a double boiler by putting 1-2 inches of water into a small saucepan over medium-low heat until the water is simmering.
Put the egg whites, sugar, and salt into a heatproof bowl and whisk them together. Put the bowl over the simmering saucepan (make sure the bottom of the bowl doesn't touch the water) and whisk constantly until the mixture reaches a temperature of 160°F / 71°C or it doesn't feel grainy when you rub it in between your fingers.
Transfer the egg white/sugar mixture to the bowl of a stand mixer with a whisk attachment fitted and whip at high speed until glossy stiff peaks form and the bowl no longer feels warm to the touch (about 10 minutes). If your meringue reaches stiff peaks and the bowl is still warm, let it sit until it cools to room temperature, then continue.
Gently fold the meringue into the larger bowl of chocolate ganache (the ganache should be cool by now; if it isn't, don't proceed until it is room temperature - you can put it in the fridge to speed this up but don't let it get too cold).Don't put it all in at once - add it in about a third at a time. Fold until incorporated and there are no lumps of meringue.
Pour the chocolate mousse into your prepared pie crust and smooth it out so it is even. Put the pie in the fridge for 10 minutes to set slightly before going to the next step.
Pour the reserved chocolate ganache on top of the mousse and spread it out so it covers the whole surface. If the ganache has cooled too much and isn't fluid enough to pour, warm it in the microwave in 5-second increments.
Chill the pie in the fridge for at least 4 hours, preferably overnight. Enjoy chilled and top with a dollop of chantilly cream.
Notes
Ingredients Notes:
Heavy Cream - You must use heavy cream or heavy whipping cream that has a fat percentage of at least 36%
Chocolate - Make sure you use real chocolate, not chocolate chips or chocolate candy bars. Look for a cocoa content of at least 54%
If these two ingredients aren't right, the pie will be soupy and won't get thick. Please pay close attention to these ingredient notes.
Egg Whites - I recommend using real, fresh egg whites rather than carton egg whites, as the carton egg whites often don't whip up very well.