In a medium saucepan over medium heat, combine the dark brown sugar, butter, and heavy cream. Stir occasionally as the butter melts to ensure everything combines evenly and doesn’t scorch.
Once fully melted and smooth, bring the mixture to a gentle simmer. Cook for 4-6 minutes, stirring frequently, until the sauce slightly thickens and deepens in color to a rich caramel brown.
Remove from heat and immediately stir in the vanilla and flaky sea salt. Mix until smooth and glossy.
Let the sauce cool for 5–10 minutes. It will continue to thicken as it sits.