Combine sugar, cocoa, flour, and salt in top of a double boiler; gradually stir in scalded milk and butter. Place over boiling water, and cook, stirring constantly, 8 minutes or until mixture is thickened.
Beat egg yolks in a small mixing bowl. Gradually stir one-fourth of hot cocoa mixture into yolks; add to remaining hot cocoa mixture, stirring constantly. Remove from heat, and stir in vanilla; let cool to room temperature.
Beat egg whites (at room temperature) until stiff peaks form; gently fold beaten egg whites into cooled cocoa mixture.
Pour into a lightly greased 1- quart soufflé dish. Place dish in a 13- x 9- x 2-inch baking pan; pour hot water into pan to a depth of 1 inch. Bake at 350° for 50 minutes or until puffed and firm. Spoon into individual serving bowls; serve immediately with sweetened whipped cream.