1. Heat chocolate, butter and water in a small saucepan over medium-low heat, stirring, until chocolate is melted; whisk until smooth. Pour into a bowl and refrigerate 30 minutes, whisking occasionally, until cool.
2. Meanwhile, arrange oven rack in center of oven and heat oven to 350 degree F. Grease a 2-1/2-quart 2-inch deep baking dish; coat with sugar.
3. Bring a large pot of water to boil. Beat yolks and 1/2 cup sugar in mixer bowl until pale yellow and thick. Beat egg whites and salt in a clean mixer bowl with clean beaters to soft peaks. Gradually beat in 1/4 cup sugar; beat until stiff. Whisk yolk mixture into cooled chocolate. Gently fold in beaten whites with a whisk, one third at a time, just until blended. Pour into prepared baking dish.
4. Place baking dish in a shallow roasting pan with at least 1 inch between baking dish and pan. Carefully pour boiling water into roasting pan to come halfway up sides of baking dish. Bake 33 to 35 minutes, until top springs back when lightly pressed in center. Remove baking dish from water bath and cool on wire rack 10 minutes. Serve warm with Sweetened Whipped Cream, if desired. Makes 12 servings.