Melt the sugar over medium to moderately high heat in a larger pot than you think you’ll need – at least a two or three quart pot, whisking or stirring the sugar as it melts to ensure it heats evenly. Cook the liquefied sugar to a nice, dark copper color.
Add the butter all at once and stir it in, before turning off the stove and pour in the heavy cream (The sauce will foam up quite a bit when you add it; this is why you want the larger pot.), whisking it until you get a smooth sauce.
You can use it right away or pour it into a jar and store it in the fridge for up to two weeks. When you take it out, it will likely have thickened a bit but 60 seconds in the microwave brings it right back to pouring consistency.