4 ounces (1 stick) unsalted butter
1 cup dark brown sugar, not too firmly packed
1 cup heavy cream
1 teaspoon vanilla extract
2 teaspoons apple cider vinegar
1/2 teaspoon salt, or to taste
In a heavy-bottomed saucepan or an enameled cast-iron pot, combine butter and sugar over medium heat and cook until sugar has melted completely and mixture has taken on a thick frothy appearance, 5 to 10 minutes. Turn off heat.
Thoroughly whisk in cream. Let cool for 10 minutes, then add vanilla, vinegar and salt. Taste and adjust seasonings.