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 over medium heat, combine sugar, water, and corn syrup. Stir continuously until the sugar completely dissolves and the mixture is clear.
03 - Increase heat to high and bring to a boil without stirring. Attach a candy thermometer and cook until the syrup reaches 300°F, the 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 just enough to create visible streaks—do not fully incorporate.
05 - Holding an apple by the stick, dip it into the hot candy syrup. Rotate and tilt to coat evenly, allowing the marbled pattern to form. Let excess syrup drip off for 5 seconds.
06 - Place the coated apple on the prepared baking sheet. Briefly swirl the remaining syrup between dips to refresh the marbled effect. Repeat with remaining apples.
07 - Let the apples stand undisturbed at room temperature until the candy shell is completely hard and set, approximately 15 minutes. Do not refrigerate.