Pour the sugar into a heavy bottomed medium saucepan and spread it out into an even layer. Turn the heat to medium. After a minute or so, the bottom layer of the sugar will begin to melt. Stir the sugar a couple times a minute with a wooden spoon or silicone spatula to combine the melted with the solid sugar. It will clump up and turn a slightly golden color. As more of the sugar melts, stir it constantly until no clumps remain and it becomes a rich amber color. Immediately pour in the cream. It will bubble violently at first and the melted sugar will seize up. Continue to stir until the caramelized sugar and the cream become a smooth, thick sauce. Turn off the heat and add the butter. Stir until the butter is emulsified into the sauce.