Line a 9-in. square pan with foil and grease foil with butter; set aside. In a large heavy saucepan, combine sugar, corn syrup and vinegar. Cook and stir over medium heat until sugar is dissolved. Bring to a boil. Cook, without stirring, until a candy thermometer reads 300° (hard-crack stage).
Remove from heat; stir in baking soda. Immediately pour into prepared pan; do not spread candy. Cool. Using foil, lift candy out of pan. Gently peel off foil; break candy into pieces.
In a microwave, melt dark chocolate coating and 1/2 teaspoon shortening; stir until smooth. Dip half the candies in the melted dark chocolate mixture, allowing excess to drip off. Place on waxed paper; let stand until set. Repeat with milk chocolate coating and remaining 1/2 teaspoon shortening and candies. 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.