As brown sugar is stored and exposed to air, it loses its natural moisture and can become hard as a rock. Discovering your brown sugar has hardened can put a dent in your baking plans. When you're in a rush, use the microwave to soften the sugar in just a few minutes. To keep the sugar from hardening in the first place, store it in an airtight container or in the freezer. Ochef suggests placing an apple slice in the bag for a few days when you first notice signs of hardening.
Measure the amount of brown sugar you need into a microwave-safe bowl. Unused sugar will harden again, so only warm the amount that you need.
Dampen two paper towels and wring out the extra water. Place the towels over the sugar and then cover the whole dish with plastic wrap.
Warm in the microwave for 30 seconds at a time, checking the softness as you go to avoid melting the sugar. The dish and the sugar will be hot, so be careful.
Fluff the sugar with a fork and then add to your recipe.