1. Move oven rack to lowest position. Heat oven to 350°F. Lightly butter 2-1/2-quart souffle dish; sprinkle with sugar. For collar, measure a length of heavy-duty aluminum foil to fit around souffle dish; fold into thirds lengthwise. Lightly butter one side of foil. Attach foil, buttered side in, around outside of dish, allowing foil to extend at least 2 inches above dish. Secure foil with tape or string.
2. Stir together cocoa, 1 cup sugar, flour and salt in saucepan; gradually stir in milk. Cook over medium heat, stirring constantly with whisk, until mixture boils; remove from heat. Gradually stir small amount of chocolate mixture into beaten egg yolks; blend well. Add egg mixture to chocolate mixture in pan, blending well. Cook and stir 1 minute. Add butter and vanilla, stirring until blended. Set aside; cool 20 minutes.
3. Beat egg whites with cream of tartar in large bowl until soft peaks form; gradually add remaining 1/4 cup sugar, beating until stiff peaks form. Gently fold about one-third of beaten egg white mixture into chocolate mixture. Lightly fold chocolate mixture, half at a time, into remaining beaten egg white mixture just until blended; do not overfold.
4. Gently pour mixture into prepared dish; smooth top with spatula. Place dish in larger baking pan; pour hot water into larger pan to depth of 1 inch.
5. Bake 1 hour and 5 to 10 minutes or until puffed and set. Remove souffle dish from water. Carefully remove foil. Serve immediately with sweetened whipped cream. 8 to 10 servings.