In a small pot, combine the brown sugar, butter, syrup and salt over medium heat and bring to a boil, stirring often to dissolve the sugar. Boil for a few minutes, swirling the pot occasionally.
Remove from the heat and whisk in the cream. Return to the heat and bring to a simmer again, whisking often. Boil for another few minutes, then remove from heat and cool to warm (you don’t want to serve molten toffee) or cool completely and refrigerate until you need it. Makes about 1 1/2 cups.