Place chopped dark chocolate and sea salt in a heat proof bowl, set aside.
Meanwhile, heat heavy cream in a small sauce pan over medium heat until steaming but do not allow to boil. You can also heat the heavy cream in the microwave in a microwave safe dish if you prefer, just do not allow it to boil.
When cream is hot, pour it over the chocolate. Allow it to sit for one to two minutes, so that the chocolate starts to melt. Carefully whisk the mixture until the chocolate is fully melted and the mixture is combined. Make sure there are no chunks of chocolate left. Whisk in vanilla extract
Place bowl in the refrigerator and chill for at least 4 hours, or overnight.
When mixture is chilled, remove from fridge and use an electric mixer to beat the mixture just until stiff peaks form. Do not over beat the mixture or the mousse will become grainy.
Portion into individual dishes. Mousse can be served right away, or chilled in the fridge until ready to serve. When ready to serve, garnish with whipped cream, chocolate shavings, or fresh fruit if desired.
Notes
I suggest using a high quality dark chocolate here - but not a super dark chocolate or the mousse might be bitter. I used a 55% dark chocolate.This makes 4 rather large servings. As it is a very rich dessert, you could get away with making it 6 servings, or maybe even 8 mini desserts.When whipping the cream, it is easiest to use an electric mixer (hand mixer or stand mixer), however, watch it carefully as it starts to thicken, and stop beating as soon as stiff peaks form. Stiff peaks means that if you turn your whisk upside down, the mousse will still stand straight up, and not bend over.