In a medium saucepan over medium heat, combine the brown sugar and unsalted butter. Stir until the butter is melted and the sugar is fully incorporated, about 2 minutes.

Increase the heat slightly and bring the mixture to a gentle boil.

Slowly whisk in the heavy cream, being careful as it may bubble up.

Continue stirring the sauce for about 5 minutes until it thickens slightly.

Once thickened, remove the saucepan from the heat.

Stir in the vanilla extract and salt.

Allow the butterscotch sauce to cool in the saucepan for a few minutes before transferring it to a heat-resistant container.

Use immediately or store in the refrigerator for later.