Heat oven to 375 degrees. Butter and sugar six 6 oz ramekins. Fill a small pot with 1 inch of water and bring to a simmer. In a metal bowl large enough to rest on the pot, combine chocolate and heavy cream. Stir until melted. Remove from heat and whisk in egg yolks one at a time, being careful not to scramble. Stir in vanilla.
In a large bowl (or a stand mixer fitted with whisk attachment) whisk egg whites on high until soft peaks form. While whisking, add sugar and salt; whisk on high until stiff peaks form.
Gently fold chocolate mixture into whipped egg whites until no streaks remain. (Do not over-mix or the soufflés won't rise to the maximum level.) Carefully spoon mixture evenly among ramekins. Bake at 375 degrees for 17 minutes, or until puffed and a cake tester inserted in the middle comes out clean. Dust with confectioners' sugar, if desired, and serve immediately.