Learn how to keep brown sugar soft and how to soften brown sugar after it has hardened.
Airtight Container
First and foremost, the best way to keep brown sugar soft in the first place is to store it the right way. When you store the sugar in a sealable, airtight container, there’s no air to absorb the moisture that causes the sugar to harden.
Sliced Bread
To keep brown sugar from drying out, turn to a loaf of bread. Throwing in a slice of white bread in your brown sugar container allows the sugar to draw moisture from it. The bread hardens while the brown sugar softens, keeping or returning it to a soft texture.
Marshmallows
How do marshmallows keep brown sugar soft, you might ask? The same way bread does! The brown sugar is able to extract moisture from the marshmallows. And a couple of marshmallows is a small sacrifice for soft, fluffy brown sugar.
Apple
If you don’t have any bread or marshmallows laying around, apples work, too. Throw in a few slices and after a day or two, the sugar will be good as new. Just be sure not to leave them in there too long.
Water
Moisture is a big deal when it comes to brown sugar. If your brown sugar has gone hard a a rock, and you can spare a few days, sprinkle a few drops of water in the bag of brown sugar and wait until it softens up. Be careful not to over do it though—water can also cause the sugar to dissolve.
Food Processor
Another helpful hack when you’re in a pinch is to use a food processor. It’ll break up the hard chunks, making the sugar usable once again. You can then use a fine mesh sieve to separate any large chunks that are leftover.
source: tasteofhome.com