Place the sugar in a medium pot with about one-quarter of its weight in water or enough to obtain a wet-sand texture. Make sure there are no sugar crystals along the inside of the pot and that all the sugar has been hydrated.
Bring to a boil over high heat. A good tip to further prevent crystallization is to cover the pot with plastic wrap; the steam that is generated will keep the sides of the pot clean of any sugar crystals.
While the sugar cooks, bring the heavy cream to a simmer.
Once the sugar reaches a dark amber color (170°C / 338°F), stir in the butter and turn off the heat.
Slowly whisk in the hot cream; it is a very violent reaction when the cream hits the sugar, so be careful to add it slowly and use a long whisk.
Pour the caramel into an adequately sized container and let the contents cool before serving. Reserve at room temperature during service; caramel has a long shelf life, but it can crystallize over prolonged periods of time. (If refrigerated, it will get too hard to portion.)