Wash thoroughly in water to remove wax coating. Dry apples thoroughly and remove stems. Insert a stick into each apple, pushing at least halfway down.
Place dry apples on parchment paper lined baking sheet.
Combine butter, brown sugar, corn syrup, sweetened condensed milk and salt in a saucepan on medium/high heat until it begins to boil. Clip candy thermometer to pan. Reduce heat to medium. Continue to stir, also scraping the sides of the pan to prevent burning.
Cooking times vary depending on the type of stove you have. Cook and stir caramel (almost constantly) until it reaches 240-245 degrees. This takes about 20 minutes on my stove, but may take more or less for you. Go by caramel temperature, NOT time. Use a candy thermometer to measure the temperature. If you stove tends to cook HOT, reduce the heat a bit. Just try to maintain a constant heat and stir frequently.
Once caramel is ready, remove from heat.
Hold onto the pan handle and tip the pan on a slight angle to allow the caramel to pool on one side. This makes dipping the apples easier. Dip apples and twirl the apple to help excess caramel drip off.
Place apples on prepared baking sheet. Allow caramel apples to cool completely before eating.