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 centered and secure.
02 - In a heavy-bottomed saucepan, combine granulated sugar, water, and light corn syrup. Place over medium heat and stir constantly until the sugar completely dissolves.
03 - Increase heat to high and bring the mixture to a boil without stirring. Attach a candy thermometer and cook until the syrup reaches 300°F, the hard crack stage. Remove from heat immediately.
04 - Stir in vanilla extract if using. Working quickly, add several drops each of red, blue, and white gel food colorings directly into the hot syrup. Gently swirl with a wooden skewer to create streaks—do not fully mix.
05 - Hold an apple by the stick and dip it into the candy syrup, turning to coat evenly. Lift and allow excess syrup to drip off. Place on the prepared baking sheet. Repeat with remaining apples, briefly swirling the syrup between dips to maintain the marbled pattern.
06 - Let the coated apples cool completely at room temperature until the candy shell hardens, approximately 15 minutes. Serve immediately or store at room temperature for up to 24 hours.