Add sugar to a large pot over medium-high heat. As the sugar melts, stir occasionally to ensure even cooking. The sugar will liquify, and then quickly start turning brown. Once the caramel turns a deep brown, almost smoking, stir in all the butter and kosher salt. Once the butter is completely melted in, take the pan off the heat, and add the heavy cream, stirring until smooth.

Use the sauce immediately or store covered in the fridge for up to 2 weeks. If the sauce has thickened after refrigeration, you can warm it in the microwave for 30 - 60 seconds to thin it out.