Caramel Sauce Hard Crack
The butter and cream will lower the temperature of the sugar to the point where it can solidify. When you add it in, you need to keep the pan over low heat and keep stirring until everything heats back up and incorporates into a smooth caramel sauce. I personally take the pan off the heat when the sugar is caramelised, add the cream and butter, and then place it back on the turned off but still warm burner while stirring. It can take as long as a couple of minutes before the sugar has melted again and you have a smooth sauce, depending on how much sauce you're making.