Prepare four 8-ounce ramekins by greasing the bottom and sides with butter. Sprinkle a little sugar on them.
In a small saucepan, heat the heavy cream over medium heat. Put the chopped chocolate into a mixing bowl. When the cream starts to simmer, immediately remove it from the heat and pour over the chocolate. Let it stand for a couple minutes and then stir it until smooth.
Add the vanilla, pinch of salt, and egg yolks to the chocolate and stir until thoroughly combined and smooth. Set aside to cool.
Preheat the oven to 425 degrees.
Put the egg whites into the bowl of a stand mixer. Whip on high, gradually adding in the sugar, until the whites form stiff peaks. This should take several minutes. Stiff peaks mean you can stick a spoon or rubber scraper in and pull it out and the egg whites form a little self-holding peak instead of just melting back into place.
Fold the egg white mixture into the cooled chocolate mixture, until it seems pretty well combined. Folding means using a rubber scraper/spatula to scrape down to the bottom and fold it over onto the top, turning your bowl as you go. You'll know it is combined when you can't see bits of the white egg anymore. Folding keeps it fluffy, while mixing would kill the fluffiness. Pour into the prepared ramekins.
Put them in the oven and close the door. Reduce the heat setting to 375 degrees. Bake for 15 minutes for small ramekins, and 25 minutes for larger ones.