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, pushing about halfway through the center.
02 - In a heavy-bottomed saucepan, combine the sugar, water, and corn syrup. Place over medium heat and stir continuously until all sugar has completely dissolved.
03 - Increase heat to high and bring the mixture to a rolling boil. Stop stirring entirely. Attach a candy thermometer and cook until the 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 directly into the hot syrup. Gently swirl with a wooden skewer to create visible streaks—do not fully incorporate the colors.
05 - Hold an apple by the stick and dip it into the candy syrup. Tilt and rotate the apple to coat it completely while preserving the marbled pattern. Lift and allow excess syrup to drip off for 5-10 seconds.
06 - Place the coated apple on the prepared baking sheet. Briefly swirl the syrup between dips to refresh the marble pattern, then repeat with remaining apples. Work quickly as the syrup will begin to thicken.
07 - Let the apples stand undisturbed at room temperature until the candy shell is completely hardened and glossy, approximately 15 minutes.