01 - Line a large baking sheet with parchment paper or a silicone mat. Insert a wooden stick into the stem end of each apple, ensuring it is secure.
02 - In a heavy-bottomed saucepan, combine sugar, water, and corn syrup. Stir over medium heat until the sugar completely dissolves.
03 - Increase heat to high and bring mixture to a boil without stirring. Using a candy thermometer, 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 a few drops each of red, blue, and white gel food colorings. Swirl gently with a wooden skewer without fully mixing to create visible streaks for the marbled effect.
05 - Holding the apples by their sticks, dip one at a time into the candy syrup. Turn to coat evenly and create a marbled pattern. Allow excess syrup to drip off.
06 - Place coated apples onto the prepared baking sheet. Repeat with remaining apples, swirling the syrup briefly between dips to maintain the marbled effect. Let cool completely until the candy shell hardens, approximately 15 minutes.