To make the caramel sauce, heat sugar in a medium saucepan over medium heat, stirring constantly. The sugar will start to clump and then melt into a thick brown, amber-colored liquid as you continue to stir. Be careful not to burn. Once sugar is melted add the butter. Be careful because the caramel will bubble when the butter is added. Stir the butter into the caramel until melted, about 2 to 3 minutes.
Very slowly, pour heavy cream into caramel while stirring. Be careful because the mixture will rapidly bubble and/or splatter when heavy cream is added. Allow the mixture to boil for 1 minute. It will rise in the pan as it boils.
Remove from heat and allow to cool to room temperature. Transfer to a jar or glass container for storage.
Notes
Store in a sealed jar in the refrigerator for up to 2 weeks.