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 sugar, water, and corn syrup. Stir over medium heat until 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 visible streaks—do not fully mix.
05 - Hold an apple by the stick and dip into the candy syrup, turning to coat evenly while maintaining the marbled pattern. Let excess syrup drip off, then place on prepared baking sheet.
06 - Briefly swirl the syrup between dips to refresh the marble pattern. Repeat dipping process with remaining apples. If syrup thickens, gently rewarm over low heat.
07 - Allow apples to cool completely at room temperature until candy shell hardens, approximately 15 minutes. Serve once fully set.