Combine the brown sugar, heavy cream, and unsalted butter over medium-low heat.
Whisk frequently for about 5 minutes at a simmer or until it begins to thicken. You want to be sure all the sugar granules have dissolved and mixture is smooth.
Remove from heat and whisk in vanilla extract and sea salt until well combined.
Allow mixture to cool completely then transfer to a small jar with a tight-fitting lid, store in the refrigerator.
Recipe by Premeditated Leftovers™ at https://premeditatedleftovers.com/?p=38233