01 - Line a large baking sheet with parchment paper or a silicone mat. Firmly press a wooden stick 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. Place over medium heat and stir continuously until all sugar crystals have completely dissolved.
03 - Increase heat to high and bring mixture to a rolling boil without stirring. Attach a candy thermometer and cook until syrup reaches exactly 300°F, the hard crack stage. Remove from heat immediately.
04 - Stir in vanilla extract if desired. Working quickly, add several drops each of red, blue, and white gel food colorings. Gently swirl with a wooden skewer to create streaks—avoid full incorporation for optimal marbling.
05 - Grasping the stick firmly, dip each apple into the hot candy syrup. Rotate continuously to ensure even coverage while maintaining the swirled pattern. Allow excess syrup to drip off for 2-3 seconds.
06 - Transfer coated apples to the prepared baking sheet. Briefly swirl remaining syrup between dips to redistribute colors. Continue until all apples are coated.
07 - Let apples stand undisturbed at room temperature for approximately 15 minutes until candy shell is completely hard and glossy before serving.