Brown sugar has a tendency to harden into a rocklike mass in the pantry. Here are some simple ways to soften it up:
For a quick fix, put the sugar in a zip-top bag and microwave it for 30 seconds; keep microwaving in quick bursts until the sugar has softened enough to scoop. Be careful not to burn it. If you can wait a day, drop a couple of apple slices or a slice of soft bread wrapped in a paper towel into a bag with the sugar and seal—the sugar will draw moisture from the apple or bread and soften up.
Best of all, store it right, so it never gets hard at all. Try one of these tricks: Store it in a zip-top bag, and put that in a sturdy plastic container with a lid that seals tight. Or use a Brown Sugar Bear, a little terra cotta device that you soak in water and put in your sugar bag to keep your sugar moist for months.
Or follow wombat’s lead and don’t buy brown sugar, just make it up as you need it by adding a small amount of molasses to granulated sugar; wombat estimates a teaspoon of molasses to a cup of white sugar approximates light brown sugar, but suggests going by color.
Board Link: Tips to loosen up brown sugar?