Instructions

1. Heat the oven to 375°F.
2. Prepare the soufflé dishes: Rub the insides of the ramekins or soufflé dish with butter. Coat with sugar by sprinkling a tablespoon of sugar in the bottom of each ramekin (or a scoop of sugar in the larger soufflé dish), and then tilting and tapping the dish to work the sugar into the corners and up the sides of the dish.
3. Melt the chocolate: Combine the chocolate and 2 tablespoons of butter in a heatproof bowl. Set the bowl over a small saucepan of barely simmering water — make sure the bottom of the bowl doesn't touch the surface of the water. (Alternatively, use a double-boiler.) Melt the chocolate, stirring occasionally, until completely smooth.
4. Cool the chocolate slightly: Remove the chocolate from heat and stir in the vanilla and salt. Let the chocolate cool until still very loose, but just slightly warm to the touch.
5. Whisk together the yolks and 1/4 cup of sugar: Transfer the yolks to a mixing bowl. Measure out 1/4 cup of sugar and sprinkle over the yolks. Whisking by hand or in a stand mixer with a whisk attachment, whisk the yolks and sugar together. They will start off bright yellow and will gradually lighten. The eggs and sugar are ready when light yellow in color, and the mixture forms ribbons that hold for a few seconds on the surface.
6. Combine the chocolate and the yolks: Pour the yolks over the chocolate. Use a spatula to gently fold the chocolate and the yolks together until completely combined.
7. Beat the eggs until frothy: Clean your mixing bowl thoroughly and make sure it is dry and free of any grease. Add the egg whites. Beat at gradually increasing speed until the whites are quite frothy and opaque.
8. Add the sugar and beat until stiff peaks form: With the mixer running at medium speed, gradually add the remaining 1/4 cup of sugar to the egg whites. Once all the sugar has been added, increase the speed to high and beat the whites until they form stiff peaks.
9. Lighten the chocolate base: Scoop about 1/4 of the beaten egg whites into the bowl with the chocolate base. Stir them in until no visible egg whites remain. This lightens the base and makes it easier to add the rest of the egg whites without deflating them too much.
10. Gently fold the egg whites into the base in two batches: Scoop half of the rest of the egg whites on top of the chocolate base. Using your spatula, cut through the center of the mixture, scoop the spatula underneath, then gently lift and flip the mixture over onto itself; this is called folding the egg whites into the base (it helps prevent deflating them too much). Give the bowl a quarter turn and repeat the folding motion. Once this batch is nearly incorporated, add the remaining whites. Continue until you see no more visible egg whites in the base.
11. Divide the soufflé batter between the prepared ramekins.
12. Bake until the soufflés are puffed and the tops look dry: Bake small soufflés for 18 to 20 minutes, or one large soufflé for 35 to 40 minutes.
13. Serve immediately!