In a medium heavy-bottomed saucepan, cook brown sugar, cream, and butter over medium heat, whisking constantly, until sugar is melted.
Remove from heat; whisk in vanilla. Whisk in confectioners’ sugar, ¼ cup (30 grams) at a time, until combined. Whisk in salt. Use immediately. (If mixture sets too quickly, it can be reheated on the stovetop or microwave in 30-second intervals.)