Combine chocolate and butter in a saucepan placed above a pot of simmering water (or use a double boiler). Stir until the butter and chocolate are one color. Remove from heat.
Put the 3 yolks in a small bowl or ramekin and add a pinch of salt – mix quickly together. Set aside.
Put egg whites (4) in a mixing bowl and using a hand mixer (or mix by hand with a whisk) mix together with 2 teaspoons of sugar (super fine if possible) until stiff peaks form (about 2-5 minutes).
Add one yolk at a time to the chocolate. Mix in each yolk until it is mixed in entirely before adding the next egg yolk.
Fold into the chocolate mixture the egg whites, combining just enough but not too much as you want to keep as much volume as possible.
Refrigerate for at least four hours or up to two days before serving. You can either keep the chocolate mousse in the saucepan and cover with plastic wrap or distribute to small bowls from which dessert will be served.
When serving, offer a small bowl of whipping cream (recipe below) for guests to add a dollop on top of their chocolate mousse.