Once you have a rich caramel immediately add the cream. Be careful in this step because the caramel will bubble rapidly when the cream is added.
Next, very slowly, drop in the butter. Since the butter is colder than the caramel, the mixture will rapidly bubble and/or splatter when added.
* The more water you use the easier it will be to control the caramel - the less water the quicker the process - do stand over caramel until it is finished.