Chop the chocolate into small pieces and put it in a large bowl and set aside.
Put the heavy cream and vanilla in a saucepan over medium heat, stirring occasionally until it is scalding hot. This means you will see steam rising from the cream and tiny bubbles appearing on the surface. Do not let it boil.
Pour the scalding cream over the chopped chocolate and let it sit for a minute. Use a metal spoon to stir in concentric circles in the middle of the bowl until all the chocolate is melted and combined with the cream. Set aside to cool while you make the meringue.
Make a double boiler by putting a heatproof bowl over a saucepan with about an inch of simmering water in it. Don't let the bottom of the bowl touch the water. Put the egg whites, sugar, and salt into the bowl and whisk together gently to combine.
Keep heating, whisking constantly but not vigorously, until the egg whites are warm and the sugar has completely dissolved. You can check this by rubbing a little bit of the mixture between your fingers - it shouldn't feel grainy at all. If you'd like to use a thermometer to make sure the eggs are completely cooked, the temperature should reach 160°F / 71°C.
Move the egg white mixture to the bowl of a stand mixer with a whisk attachment fitted and whip on medium speed for a few minutes until the mixture cools slightly and begins to thicken.
Gradually turn the mixer up to high speed and let it whip until glossy, firm peaks form. This means the meringue makes tall peaks that still droop slightly at the top. Don't whip them all the way to stiff peaks or the meringue may not incorporate into the mousse smoothly.
Make sure your chocolate ganache has cooled to room temperature. If it hasn't, let it cool a little longer before proceeding.
Add about a third of the meringue to the chocolate ganache and fold it in gently with a rubber spatula. Then fold in the rest in two more additions. Only fold until incorporated - be careful not to overmix or deflate the egg whites.
Spoon or pipe your chocolate mousse into serving dishes. It works great in ramekins, coup glasses, or even dessert rings.
Chill for at least 4 hours. Before serving, top with chantilly cream and chocolate curls.
Notes
Ingredients Notes:
Egg Whites - You only need the egg whites for this recipe (not the yolks). For best results, use fresh eggs and separate them, rather than using carton egg whites, as boxed egg whites often don't whip up as well. Be very careful when separating your eggs as even a tiny trace of egg yolk in the whites will stop them from whipping up properly.
Sugar - Regular white granulated sugar.
Chocolate - It's important to use real, good-quality chocolate, not chocolate chips or candy bars. I recommend dark chocolate or bittersweet chocolate for the best flavor, ideally with at least 54% cocoa solids. You can use milk chocolate, but the flavor won't be as rich.
Cream - Use heavy cream, heavy whipping cream, or double cream. It should have at least 36% fat content.