Sprinkle with nuts or whatever you desire while the caramel is still warm (work quickly, the caramel sets VERY fast).
Bring a large saucepan of water to a boil. Dip apples in boiling water for a few seconds, then dry with paper towels to remove any wax from the peels. Set apples aside; when completely cool, insert a wooden chopstick firmly into the bottom of each apple for a handle.
Line a baking sheet with parchment paper.
Stir butter, brown sugar, corn syrup, and sweetened condensed milk together in a saucepan over medium-high heat. Bring to a boil, stirring constantly, and reduce heat to medium. Cook until a candy thermometer reads 248 degrees F (120 degrees C), 25 to 30 minutes; stir constantly to prevent burning. A teaspoon of the syrup, dropped in a glass of cold water, should form a firm ball. Remove caramel from heat and stir in vanilla extract.
Working quickly, dip each apple into the hot caramel to completely coat the apple and about 1/2 inch of the wooden handle. Sprinkle apples with any desired toppings while still hot. Let caramel apples cool on the parchment paper. Use caution, caramel is very hot.
Because the caramel requires constant attention when cooking and you will be in a hurry when coating the apples, prep everything beforehand. Have your parchment paper laid out, your toppings ready in bowls, etc. Also, each stove differs as far as temperature and how long you will need to cook caramel. It only took 21 minutes for me, but took a friend 39 minutes, so it can vary greatly.