In a medium saucepan, melt the butter over medium heat. 

Add the brown sugar and lemon juice, stirring to combine. 

Bring the pan to a boil, then reduce heat to a steady boil. Stir frequently for 2-3 minutes until the mixture is smooth. 

Be careful as you add the heavy cream to the saucepan. Stir to combine and return the pan to a steady boil. Cook for 4 minutes, stirring frequently. 

Add the vanilla, whiskey (if using), and salt. Stir to combine. 

Allow sauce to cool slightly.

Transfer to a glass jar and allow to cool completely. The sauce will thicken as it cools. 

Rewarm sauce before serving.