Line a cookie sheet with parchment paper and set aside.
Wipe down apples with a mixture of the white vinegar and 1 cup of hot water. Insert one cookie stick parallel to the stem of each apple. Place apples on prepared cookie sheet to dry completely.
In a medium saucepan, melt the butter over medium heat.
Add the brown sugar, corn syrup, and salt (if using.) Stir and cook until mixture reaches a full rolling boil.
Place a candy thermometer into the saucepan and clip to the side so that you can monitor continually.
Add sweetened, condensed milk to the mixture and continue cooking and stirring until mixture reaches exactly 248 degrees on the thermometer.
Remove from heat and stir in the vanilla extract.
Working quickly, dip each apple into the caramel and swirl gently to coat. Allow excess caramel to drip off the apple; then, place apple onto the parchment paper/cookie sheet to cool.
The caramel will cool within minutes and the apples are ready to eat! Bite into apples right on the stick or slice and serve.
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