Melt 1/2 cup of unsalted butter in a
heavy-bottomed, small saucepan over medium heat. Add 1 cup of packed light or dark brown sugar and 1 tsp fine salt and stir vigorously until combined. Continue to stir the mixture constantly as it cooks. If you don't, it can burn quite quickly! If the mixture separates while it's cooking, remove it from the stovetop and stir vigorously until it comes back together. Then return it to heat and continue cooking it.