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 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 300°F, the hard crack stage. Remove from heat immediately upon reaching temperature.
04 - Stir in vanilla extract if using. Quickly add several drops each of red, blue, and white gel food coloring. Gently swirl with a wooden skewer to create streaks—do not fully mix. The goal is visible color separation for the marbled effect.
05 - Hold an apple by the stick and dip into the hot candy syrup. Rotate and tilt to coat evenly while creating marbled patterns. Lift and allow excess syrup to drip off for a few seconds.
06 - Place coated apple on prepared baking sheet. Briefly swirl remaining syrup between dips to refresh the marble pattern. Repeat with remaining apples, working quickly before syrup thickens.
07 - Allow apples to cool completely at room temperature until candy shell is fully hardened, approximately 15 minutes. Do not refrigerate as condensation can ruin the glossy finish.