Brown sugar becomes hard when moisture in the sugar evaporates. Therefore, the various methods used for softening brown sugar are intended to return moisture to the sugar. We recommend you try the following methods for softening brown sugar that has hardened:
Quick softening method: If you need to use hard brown sugar immediately, remove it from the package into an oven-safe container and heat it at 250°F. Watch it carefully. As soon as it’s soft, measure the amount you need right away because it will again harden as it cools. Please use caution. Oven heated sugar is very hot!
Microwave softening method: Place about 1/2 lb. of hardened brown sugar in microwave-safe bowl. Cover sugar with two pieces of wet (but not dripping) white paper towels. Tightly cover bowl with plastic wrap. Heat in microwave at HIGH for 1 1/2 - 2 minutes. Divide sugar with fork (sugar will be hot); stir. Again, microwave-softened sugar hardens as it cools, so microwave only the amount of sugar you need. Use immediately and use caution as it will be hot. Microwave ovens vary in power; cooking time may need adjustment.
Time-permitting softening method: Place hardened brown sugar in a tight sealing container. Cover sugar with two pieces of dampened – not dripping – wet paper towels placed over a small piece of plastic wrap or foil on top of the sugar. Or you can add a slice of bread to the bowl of sugar. Place lid on container and seal tightly. Remove the paper towel or bread after the sugar absorbs the moisture and softens (about two days) and tightly reseal the container. Divide sugar with fork; stir. Sugar should remain soft.