I often make this in the fall and winter months, and serve it in little dessert glasses. It's delicious served with raspberries and a fresh mint leaf, or crushed pistachios and a dollop of whipped cream.
Using the double boiler method, melt the chocolate and butter together. Be sure not to let any moisture/water/steam into your chocolate as it will cease and ruin the batch. Once the chocolate and butter have melted, remove from the heat, and stir gently using a rubber spatula. Set aside and allow to cool.
In two mixing bowls, separate the eggs- whites in one bowl, yolks in the other bowl. The egg yolks should be in a larger bowl, because you will eventually add all of your ingredients into it.
Add 1/4 cup of sugar to the bowl with the eggs yolks and whip on medium speed about 5-7 minutes, until light yellow and a bit fluffy.
Using a clean, dry whisk attachment, now whip the egg whites. Once they have reached soft peaks slowly add in the sugar, then the salt and vanilla. Whip until stiff peaks form, and take care not to over whip them.
Pour the cooled chocolate mix into the egg yolk/sugar mixture (It will probably still be warm, which is okay. As long as it's not hot.) Using a rubber spatula, gently fold in to fully combine, until there are no yellow streaks. Once fully combined, fold in the egg whites until there are no white sections and everything is combined.
Divide the mousse into small, 4-6 ounce serving dishes, and allow to chill in the fridge for at least a few hours before serving. You can also put all of the mousse into a larger tupperware container, and refrigerate for at least a few hours, and then spoon into bowls, on tart shells, or however you please. Serving with fresh whip cream is highly recommended!
Notes
This rich, fluffy dessert is best made the morning of or the day before. It will keep in the fridge for about 3-4 days.