01 - Line a large baking sheet with parchment paper or silicone mat. Insert wooden sticks into stem end of each apple, ensuring secure placement.
02 - Combine sugar, water, and corn syrup in heavy-bottomed saucepan. Stir over medium heat until sugar completely dissolves.
03 - Increase heat to high and bring mixture to boil without stirring. Attach 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 wooden skewer—do not fully incorporate. Visible streaks create marbled effect.
05 - Hold apple by stick and dip into candy syrup, turning to coat evenly. Lift and allow excess syrup to drip off. Place on prepared baking sheet. Swirl syrup briefly between dips to maintain marble pattern.
06 - Let apples cool completely at room temperature until candy shell hardens, approximately 15 minutes. Serve once set.