Wash the apples thoroughly, removing any wax from the skin if necessary. Dry them completely with paper towels, remove the stems, and insert a sturdy wooden stick into the top of each apple. Line a large baking sheet with parchment paper and arrange your sprinkles in small bowls nearby.
Add the granulated sugar, water, corn syrup, and white vinegar to a clean, heavy-bottomed saucepan. Place over low heat and stir gently until the sugar has completely dissolved. Avoid allowing the mixture to boil before the sugar crystals disappear.
Once the sugar has dissolved, bring the mixture to a boil. Attach a candy thermometer to the saucepan and cook without stirring until the syrup reaches 300–302°F, the hard-crack stage. If sugar crystals form along the sides, carefully brush them down with a pastry brush dipped in water.
Remove the saucepan from the heat once the syrup reaches the correct temperature. Carefully stir in black gel food coloring until the coating reaches a deep, dramatic shade. Add the coloring gradually because the hot syrup may bubble when it is mixed.
Tilt the saucepan slightly to create a deeper pool of candy coating. Holding each apple by its stick, dip it into the hot syrup and rotate it until the surface is evenly covered. Lift it out, let the excess coating drip back into the pan, and gently rotate the apple to remove extra drips.
While the coating is still sticky, roll the lower portion of each apple in Halloween sprinkles, or add decorations of your choice. Work quickly because the candy shell begins to harden within minutes. Place each apple on the prepared baking sheet with the stick pointing upward.
Allow the apples to cool at room temperature for about 20 minutes, or until the coating is completely hard and crisp. Once set, serve them as they are or wrap each apple individually in cellophane for a festive Halloween treat.
