Step 1: Wash and thoroughly dry the apples. It is crucial that there is no moisture or wax on the apples for the toffee to stick. Insert a wooden skewer firmly into the stem end of each apple, pushing about halfway through.
Step 2: Prepare a baking sheet by lining it with parchment paper or a silicone mat and lightly greasing it with a neutral oil or non-stick spray.
Step 3: In a heavy-bottomed saucepan, combine the granulated sugar, light corn syrup, water, and white vinegar. Stir gently just until the sugar is moistened.
Step 4: Attach a candy thermometer to the side of the pan, ensuring it does not touch the bottom. Cook over medium-high heat without stirring.
Step 5: As the mixture heats, occasionally dip a clean pastry brush in water and brush down any sugar crystals that form on the sides of the pan. This prevents crystallization.
Step 6: Continue cooking until the candy thermometer registers 300-310°F (149-154°C), which is the 'hard crack' stage. This usually takes 15-20 minutes.
Step 7: Once the desired temperature is reached, immediately remove the pan from the heat. Carefully stir in the unsalted butter and black gel food coloring until fully incorporated and the mixture is a deep, glossy black.
Step 8: Working quickly, tilt the saucepan to one side. Dip one apple into the hot toffee, rotating it to fully coat the apple. Lift the apple and allow any excess toffee to drip back into the pan. You can gently scrape the bottom of the apple against the rim of the pan to remove any large drips.
Step 9: Place the coated apple onto the prepared baking sheet. Repeat with the remaining apples, working as quickly as possible before the toffee cools and hardens.
Step 10: Allow the Poison Toffee Apples to cool completely at room temperature for at least 30 minutes to an hour, or until the toffee is hard and set. Do not refrigerate, as this can cause the toffee to become sticky.