To soften hardened brown sugar that you want to use in cookies, put the brown sugar in a microwave-safe bowl and place a damp paper towel over top of it. Microwave it in 10-15 second increments, pausing in between to poke the sugar with a spoon. Continue to do this until enough of the sugar has softened and it becomes usable. To prevent this from happening in the first place, I swear by these brown sugar bears. You'll never find yourself dealing with hardened brown sugar again!