Cook sugar, corn syrup and vinegar in heavy saucepan over medium heat, stirring constantly until sugar dissolves, 3 to 4 minutes. Stop stirring and bring candy to boil and continue cooking without stirring until candy reaches hard-crack stage (300 to 310 degrees on candy thermometer), about 10 minutes. Do not overcook. Remove candy from heat and quickly stir in baking soda.

Pour into buttered 13- by 9-inch baking pan but do not spread (candy wont fill pan). Let candy cool. Melt chocolate in top of double boiler over simmering water and set aside to cool. Break hardened candy into chunks and dip into melted chocolate. Place on wax paper until firm, about 30 minutes, or refrigerate. Store in airtight container. Yields 2 1/2 pounds. Each serving: calories; 313 mg sodium; 20 mg cholesterol; 0 fat; 16 grams carbohydrates; 48 grams protein; 0.45 gram fiber