Combine first 3 ingredients in a large heavy bottomed sauce pan over medium-high heat. Stir to combine.
When Butter is melted, add in the condensed milk. Attach the candy thermometer and mix constantly until it reads 240 (Or Soft Ball Stage) I think this varies between 230-240 degrees.
When caramel is at the right temperature, remove from heat and whisk in the vanilla.
Pour into prepared baking dish and sprinkle with sea salt (as liberally as you want.) Allow caramel to cool in the fridge for about 30-45 minutes.
When caramel is set, use a butter knife or spatula to help wedge the entire sheet of caramel onto a cutting board. Cut into strips and then layer two strips and cut them into small pieces. Wrap with squares of wax paper. Enjoy!
Recipe by The Baker Chick at https://www.thebakerchick.com/sea-salt-caramels/