Add sugar to a saucepan, heat over medium-low. Allow the sugar to dissolve in a few minutes.
Increase the heat to medium high. Cook the sugar to caramelize by stirring until it turns medium brown (not too dark). No water needed in this step, the sugar will dissolve by itself. It would take 5-6 minutes for the caramel to reach 330F. Then reduce the heat to medium-low.
Add the butter into the sauce. Bubbles may come in this step but keep whisking constantly and gently. The butter will stop the caramel cooking process and leave the smooth texture of the sauce. In this step you would find the color of caramel a bit lighter.
Remove from the heat. Carefully add salt and the heavy cream and stir until the sauce smooths out.
Let the sauce cool a bit and when it's still warm, then transfer to a jar or container. Store at room temperature or chilled in the fridge. Reheat for a few seconds before serving.