3 tablespoons (about) unsalted butter, melted, cooled
1/3 cup unsweetened cocoa powder
The soufflés can be assembled up to 1 day ahead, covered and refrigerated. Bake a couple of minutes longer.
Coat the interiors of six 7.75-ounce soufflé dishes completely with some of the butter. Refrigerate until the butter is cold and set, about 2 minutes.
Brush a second coating of butter over the interiors of the dishes with your brush strokes going from the bottom of the soufflé dish to the top. Coat the interiors completely with sugar and dust off excess. Place the dishes on a baking sheet
Using an electric mixer, beat the egg whites in a large bowl until they are foamy. Gradually beat in 1/3 cup of sugar. Continue beating until the egg whites are shiny and form medium-soft peaks when the beaters are lifted.
Meanwhile, stir the chopped chocolate in a large bowl set over a saucepan of simmering water until melted and smooth. Remove from the heat. Add the water and cocoa powder and mix until smooth. Fold a fourth of the egg white mixture into the chocolate mixture. Fold in the remaining egg white mixture (the mixture will resemble chocolate mousse).
Divide the soufflé batter equally among the prepared soufflé dishes. Using a large knife, level the filling flush to the top of the dishes. Using a clean cloth, wipe the very top edges of the dishes.
Bake the soufflés on the baking sheet until they puff but are still moist in the center, about 12 minutes. Serve immediately.