In a 2nd sauce pan, combine the cream, butter, vanilla, and kosher salt. Heat over medium heat. Once the mixture reaches a simmer, remove from heat and set aside. Slowly add the cream mixture to the sugar mixture. It will bubble violently so be careful. Turn the heat back on and continue cooking until the mixture reaches 245 degrees on a candy thermometer. If your mixture becomes any hotter the candy will be too firm, any cooler – it won’t set. Watch it closely. It takes about 12 minutes. Pour out the mixture into the prepared pan and pop in the fridge for an hour to cool.