01 - Line a large baking sheet with parchment paper or a silicone mat. Insert a wooden stick firmly into the stem end of each apple, pushing about halfway through the fruit.
02 - In a heavy-bottomed saucepan over medium heat, combine sugar, water, and corn syrup. Stir continuously until all sugar crystals have completely dissolved.
03 - Increase heat to high and bring mixture to a rolling boil. Stop stirring completely. Attach a candy thermometer and cook until syrup reaches 300°F, the hard crack stage. Remove from heat immediately.
04 - Stir in vanilla extract if desired. Working quickly, add several drops each of red, blue, and white gel food colorings directly into the hot syrup. Gently swirl once or twice with a wooden skewer to create streaks—do not fully incorporate for distinct marble patterns.
05 - Hold an apple by the stick and dip into the candy syrup, rotating completely to coat. Lift and allow excess syrup to drip off for 2-3 seconds. Place on prepared baking sheet. Briefly swirl syrup between apples and repeat with remaining apples.
06 - Let apples stand undisturbed at room temperature until candy shell is completely hard and glossy, approximately 15 minutes. Serve immediately or store at room temperature up to 24 hours.