Remove apples from refrigeration 30 minutes before starting. Wash thoroughly, dry completely, and twist off stems. Insert wooden sticks firmly into the stem ends, pushing about halfway through each apple. Line a baking sheet with parchment paper and lightly coat with cooking spray.
In a small bowl, combine 1½ tablespoons of Tajin seasoning with the lime zest and cayenne pepper (if using). Set aside.
In a medium heavy-bottomed saucepan, combine sugar, water, and corn syrup over medium heat. Stir gently until sugar dissolves, then increase to medium-high heat. Stop stirring completely once the mixture begins to boil. Allow the mixture to cook undisturbed until it reaches a deep amber color (approximately 350°F on a candy thermometer).
Remove pan from heat and carefully add the room temperature heavy cream—it will bubble vigorously, so stand back! Whisk until smooth, then add butter, vanilla, and your prepared Tajin-lime mixture. Return to low heat and stir continuously for 2 minutes to fully incorporate flavors.
Allow caramel to cool slightly (about 10 minutes) until it reaches 200°F or has a thick, syrupy consistency. Holding an apple by its stick, dip and twirl it in the caramel, allowing excess to drip back into the pan. After coating, hold the apple upside down for 20 seconds to prevent pooling at the bottom.
While the caramel is still tacky but not dripping, immediately sprinkle the remaining Tajin seasoning over each apple, focusing on the upper half for an attractive gradient effect. Place finished apples on the prepared baking sheet.
Refrigerate for at least 1 hour to set completely before serving.