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, ensuring it's secure for dipping.
02 - In a heavy-bottomed saucepan, combine granulated sugar, water, and light corn syrup. Stir over medium heat until the sugar completely dissolves.
03 - Increase heat to high and bring mixture to a boil without stirring. Attach a candy thermometer and cook until syrup reaches 300°F (hard crack stage). Remove from heat immediately.
04 - Stir in vanilla extract if using. Quickly add several drops each of red, blue, and white gel food colorings. Gently swirl with a wooden skewer to create streaks—do not fully mix.
05 - Holding an apple by the stick, dip it into the hot candy syrup, turning to coat evenly. Lift and allow excess syrup to drip off. Place on prepared baking sheet.
06 - Repeat with remaining apples, swirling syrup briefly between each dip to refresh the marbled pattern. If syrup thickens, gently rewarm over low heat.
07 - Let apples cool completely at room temperature until candy shell hardens completely, approximately 15 minutes. Serve once set.