To start, prepare a hot water bath ready for use after the ganache has been made.

To make the ganache, place the chopped chocolate into medium heatproof bowl.
Combine the milk and salt in a small saucepan over medium heat. Bring to a low boil. Wait a moment until the steam dissipates; then, add the vanilla and immediately pour over the chocolate.

Cover the bowl with a plate and allow to set for 3 minutes. Remove the plate and stir until the chocolate is completely melted. Set over the hot water bath to keep the chocolate liquid while you make the meringue. Do not let the chocolate get hot.

Place the chilled aquafaba into the bowl of a standing electric mixer fitted with whisk attachment. Add cream of tartar and mix on low speed for 1 minute. Increase the speed to medium-high and beat for 7 minutes or until the meringue is white, thick, glossy and greatly expanded. Lift the whisk; the meringue should hold peaks and not droop or fall off.

With mixer on low, sprinkle in the sugar, 1 tbsp at a time, increasing speed to high and beating for 30 seconds after each addition, and beating 30 seconds between additions. Make sure that you lower the speed for each addition. When all of the sugar has been added, scrape down the sides of the bowl and continue to beat for an additional 5 minutes or until stiff peaks form.
While the meringue is beating, remove the bowl of chocolate from the water bath. Wipe the bottom of the bowl. Stir gently until the chocolate is at room temperature.

Fold the chocolate into the meringue. First, lighten the mixture by gently folding a large scoop (about 1/3 of the total) of the meringue into the chocolate using a rubber spatula.

Add the remaining meringue and gently plunge the spatula down (hold it so that the flat side is facing you in the center of the mixture). Bring it up towards the top of the bowl, scooping a good amount of meringue with it. Bring the spatula up the side of the bowl and then down again into the center of the meringue again.

Repeat rotating the bowl, working your way around the bowl, always starting in the center, and moving towards the side of the bowl.

Repeat the folding movement until the mixture is more or less homogenous. Don’t over mix. You can stir in any large clumps of meringue if needed. If you are adding the liqueur, gently fold it in now.

Spoon, pour or pipe the mousse into individual serving dishes. Transfer to the refrigerator for about 15 minutes or until set. The mousse sets quickly and is usually ready to serve in as little as 15 minutes, but it best to allow more time for it to chill thoroughly..

Garnish with grated chocolate, a dollop of any cream, a few berries or a drizzle of a chocolate sauce.

This mousse will keep for up to 3 days in the refrigerator. It can also be frozen. It will thicken more, but it will not harden completely. Frozen mousse tastes like a delicious creamy ice cream. Defrost in the refrigerator before serving.