Core each apple and insert a stick or skewer into the stem end, then set them aside on a parchment-lined baking sheet.
Combine sugar, water, corn syrup, and salt in a heavy-bottomed saucepan over medium heat, stirring gently until the sugar dissolves completely. Bring the mixture to a boil without stirring and insert the candy thermometer.
Cook the mixture until it reaches 300°F (hard crack stage), which takes about 10-15 minutes. Observe the color—it should turn a deep amber or dark caramel hue. Remove from heat immediately to prevent burning.
Quickly stir in vanilla extract and food coloring, then carefully tilt the pan to coat the sides if needed to mix the color evenly. The toffee will be hot and bubbling—use caution.
Dip each apple into the hot toffee, tilting the pan to help the glaze flow evenly, then lift out and allow any excess to drip off. Place the coated apples on the prepared baking sheet to cool and harden.
While the toffee is still warm and flexible, add edible decorations on top for a spooky, ghoulish effect.
Allow the apples to cool completely at room temperature until the toffee is shiny, firm, and cracks slightly when gently pressed—a sign it's ready.
Once cooled and hardened, serve the Poison Toffee Apples whole or cut into pieces for a dramatic presentation.