Place the sugar in a saucepan 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.
While the sugar cooks, bring the heavy cream to a simmer.
Cook the sugar to 170°C / 338°F or until it has a dark amber color. Turn the heat off and slowly stir in the hot cream in increments, using a long whisk. Once all of the cream has been incorporated, turn the heat up to high. Stir in the butter and turn off the heat.
Pour the caramel into an adequately sized container and let the contents cool to room temperature.
Reserve at room temperature during service (if refrigerated, it will get too hard to portion). It can last for up to 1 month if it is kept covered and in a cool area.