Grease a 13-in. x 9-in. pan with butter; set aside. In a large heavy saucepan, combine the sugar, corn syrup and vinegar. Cook and stir over medium heat until sugar is dissolved. Cook, without stirring, until a candy thermometer reads 290° (soft-crack stage).
Remove from the heat; stir in baking soda. Pour into prepared pan but do not spread; cool. Break into pieces.
Dip each candy halfway into white candy coating; allow excess to drip off. Place on waxed paper-lined baking sheets until set. Dip plain half of candies into dark chocolate coating; allow excess to drip off. Return to waxed paper until set. Store in an airtight container.
We recommend that you test your candy thermometer before each use by bringing water to a boil; the thermometer should read 212°. Adjust your recipe temperature up or down based on your test.