Wash the apples thoroughly and remove the stems. Insert a skewer or popsicle stick into the core of each apple to make handling easier. Place the prepared apples on a tray lined with parchment paper.
Combine granulated sugar, water, and butter in a heavy saucepan over medium heat. Stir gently until the sugar dissolves completely and the mixture begins to bubble.
Attach a candy thermometer and cook the toffee mixture without stirring until it reaches 150°C (300°F), indicating it has turned into a caramel toffee consistency. Watch for a deep amber color and a fragrant caramel aroma.
Remove the saucepan from heat and carefully dip each apple into the hot toffee, tilting the pan as needed to submerge the apple completely. Tap gently to let excess toffee drip off, then place the coated apple on the parchment-lined tray to set.
Repeat the dipping process for all apples, ensuring an even, glossy coating. Allow the toffee to cool and harden, which should take about 10-15 minutes at room temperature.
Once the toffee is firm and shiny, serve the Poison Toffee Apples as a spooky, crunchy treat perfect for Halloween celebrations.
