Preheat oven to 375°. Grease the inside of four 6-ounce ramekins well with butter including the top of the outside rim. Sprinkle a little sugar in each and tap it around to coat the inside well. Dump out any excess sugar. Place on a small baking sheet.
Create a double boiler by placing a large bowl over a pot of simmering water, ensuring that the bottom of the bowl does not touch water. Add chocolate and butter to bowl and stir until completely melted. Remove from heat and stir in vanilla and salt. Let cool while you whip egg whites.
Add egg whites to a medium bowl and beat with a hand mixer. Once at soft peaks start adding sugar slowly, 1 tablespoon at a time. Continue beating until egg whites reach stiff peaks.
Add egg yolks one at a time to cooled chocolate, whisking well after each one. Add about ½ cup of whipped egg whites to chocolate and stir until just combined. Add the remaining egg whites in 2 batches, gently folding in after each addition just until no white streaks remain.
Gently pour batter into prepared ramekins, filling just below the top rim and smooth tops.
Bake until soufflés have risen tall and are just slightly wobbly, about 20 minutes.
