You can keep brown sugar soft in advance with the help of bread or marshmallows, but if you forget or just have some hard brown sugar lying around that you need to soften right now, redditor whitehandsinkstains notes you can solve the problem with a little water and a microwave.
- Put the rock-solid mass of sugar in a big, microwave-safe bowl. Fill another little bowl, or a coffee mug, with warm water. Put both these things in the microwave.
- Microwave in 30 second increments until the sugar is just soft enough to break off chunks. (This usually takes me ~2-3 minutes, depending on how hard the sugar is.)
- Break off more sugar than you need (you can pack it into a measuring cup and get a good loose estimation) and stash the rest. If you can, break your target sugar into even smaller chunks before tossing it back in the microwave, at 30 second increments again.
- Once it’s suitably soft, you might notice lingering nuggets that just won’t soften. Two options here — if you’re making cookies, nuggets of brown sugar turn into fantastic caramel, so you can leave them. Otherwise, crumble things down as much as you can and fish out the hard bits.
As noted, the process isn’t perfect but it does make a notable difference (and is much less stressful than stabbing the clumps with a wooden spoon for hours). I’ve done this a few times when baking and it definitely helps. I’ve found it’s best to microwave on lower heat and, as whitehandsinkstains, break the process into 30 second increments. Microwaving your sugar for too long when it’s too hot can provide undesirable effects.