Insert skewers or apple sticks into the stem end of each apple, pressing them firmly but gently so they stay upright.
Line a baking sheet with parchment paper and set aside to prepare for cooling the coated apples.
Combine sugar, water, and cream of tartar in a heavy-bottomed saucepan and attach a candy thermometer to the side.
Heat over medium heat without stirring until the sugar syrup begins to boil and reaches 300°F (hard crack stage), watching for a deep amber color and a bubbling, syrupy texture.
Remove the saucepan from heat and carefully stir in red food coloring for a sinister, blood-red hue, then swirl gently to mix evenly.
Dip each apple into the hot caramel, tilting and rotating to coat completely with a glossy, crimson shell. Let the excess drip off before placing the apple on the prepared parchment.
Allow the apples to cool and the coating to harden at room temperature for about 15-20 minutes, until the shell is crisp and shiny.
Optionally, dust the finished apples with powdered sugar for a ghostly, sickly appearance before serving.
Once set, serve these creepy Poison Toffee Apples as a spooky treat for Halloween parties or haunted gatherings.