In a medium saucepan over medium heat, add the unsalted butter. Allow it to melt completely, swirling the pan occasionally to ensure even melting.
Once the butter is melted, add the packed dark brown sugar and salt to the saucepan. Stir continuously until the mixture becomes smooth and starts to bubble, about 2-3 minutes.
Slowly pour in the heavy cream while stirring. Be cautious, as the mixture will bubble up when the cream is added. Continue to stir until the sauce is smooth and thickened, about 5 minutes.
Remove the saucepan from heat and stir in the vanilla extract. This will enhance the flavor and aroma of your Easy Butterscotch Sauce.
Allow the sauce to cool slightly before transferring it to a heat-resistant container. It can be stored in the refrigerator for up to two weeks.