Now, the sort of hard part. You need to add the cream. Because the cream is cooler than the sugar and butter, it will have several effects on the mix. First, it might make the mixture splatter and bubble up. Second, it might make the mixture harden without the cream being incorporated. That's not what you want obviously. The caramel should be pourable when finished. If it has hardened without the cream being incorporated, try again. This takes some practice to get right, so don't be discouraged if it doesn't work on your first attempts.