Every time I have tried to make caramel sauce (with the wet method) it has been going fine during carmelization, but then everything just seizes up into a rock when I try add in the cream and butter. This makes sense, since carmelization happens above the hard crack stage, but no recipe ever mentions it, just crystallization of the syrup during carmelization. Is it something I am doing wrong, or do they just never mention that for some reason?
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.