In a medium saucepan, add in the sugar in an even layer. Heat the sugar over medium-high heat, stirring with a whisking as it begins to melt. ( The sugar may start forming clumps, it's ok, just keep stirring. As it cooks, it will melt back down.) Stop whisking once all of the sugar has melted, and swirl the pan occasionally while the sugar cooks.

Continue cooking until the sugar has reached a deep amber color. It should look almost a reddish-brown, and have a slight toasted aroma. ( This is the point where caramel can go from perfect to burnt in a matter of seconds, so keep a close eye.) If you are using an instant-read thermometer, it will be around 350 degrees F.

As soon as the caramel reaches its sweet spot, add the butter all at once. **Be careful, as the caramel will bubble ferciously when the butter is added.** Whisk constantly the butter into the caramel until it is completely combined. Remove the pan from heat.

Slowly pour the cream into the caramel. Again, be careful because the mixture will once again bubble up.

Whisk until all of the cream has been incorporated and you have a smooth sauce. (the caramel will become super sticky and stringy when adding the cream, just keep stirring. It will eventally incorporate all together).  

Add the kosher salt  and stir with whisk until combined. Let it cool about 15 minutes before pouring into glass jar to give as a gift or in an air tight container to put into the fridge.  

It can be stored in the fridge for up to one month.