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 it about halfway through. Ensure apples are completely dry to help the candy adhere properly.
02 - In a heavy-bottomed saucepan, combine granulated sugar, water, and light corn syrup. Place over medium heat and stir constantly until the sugar has completely dissolved and the mixture is clear.
03 - Increase heat to high and bring the mixture to a rolling boil. Stop stirring and attach a candy thermometer to the pan. Cook until the syrup reaches exactly 300°F, the hard crack stage. Remove from heat immediately to prevent burning.
04 - Stir in vanilla extract if using. Working quickly, add several drops each of red, blue, and white gel food coloring directly into the hot syrup. Use a wooden skewer to gently swirl the colors—do not mix completely. You want distinct streaks visible for the marbled pattern.
05 - Hold an apple by the stick and dip it into the candy syrup, tilting the pan as needed to submerge the apple completely. Rotate the apple to ensure even coverage and allow excess syrup to drip off for a few seconds. The swirling motion in the syrup will create the marbled pattern on the apple.
06 - Place the coated apple onto the prepared baking sheet. Briefly swirl the remaining syrup between dips to refresh the marbled effect. Repeat with remaining apples. If the syrup becomes too thick, gently rewarm over low heat for 30 seconds.
07 - Let the apples sit undisturbed at room temperature until the candy shell has completely hardened and set, approximately 15 minutes. Once set, the apples are ready to serve or can be stored at room temperature for up to 24 hours.