In a heavy saucepan, combine the butter, brown sugar, corn syrups, sweetened condensed milk, and salt; bring to a boil over medium-high heat.
Cook and stir until the candy thermometer reads 248 degrees. It can take 30-40 minutes
Remove from heat and stir in vanilla.
You can dip your apples right away, but I have found the caramel then drips down the sides of the apple a bit more. I let mine cool to 150. I’d probably suggest letting it cool to 175-200 because mine was a tad thick.