In a perfect world, you would've stored your brown sugar in an airtight container and prevented your brown sugar from getting hard in the first place. But this is not a perfect world, and not all of us have airtight containers ready to go. So don't feel ashamed if you find yourself frantically searching "how to soften brown sugar" on Google as a batch of muffin batter sits on your counter and your oven preheats to 350ºF. We've all been there. Fortunately, figuring out how to soften brown sugar isn't that hard. (Pun intended.) There are a few different ways you can soften hard brown sugar, all of which are fairly simple.
The quickest way to soften brown sugar? Zap it in the microwave, in ten second bursts. That way, the sugar doesn't accidentally melt and caramelize. The writers at Epicurious also recommend putting a damp paper towel over the microwave-safe bowl full of hard brown sugar so it doesn't dry out. If you have a little bit of time, and the right tools, the experts at Bon Appétit suggest softening brown sugar by "giving clumpy brown sugar a whirl in the food processor or high-powered blender."
If you don't have a microwave, a food processor, or a high-powered blender, you can also soften hard brown sugar in the oven. Teresa Floyd wrote for Food52, you can take a big block of hardened brown sugar, plop it on a cookie sheet covered in parchment, bake in the oven at 250ºF for five minutes, then break it up with a fork. Folks on Pinterest seem to like covering a baking pan with aluminum foil and baking the hardened brown sugar at 300ºF for five minutes, presumably to keep the moisture in.
If you're not in a rush and just want to bring hard brown sugar back to life, you have a few more options. Take the hunk of brown sugar, and place it in a zippered plastic bag with a slice of bread or a slice of apple or even a marshmallow. Let it sit for a day or two, and the moisture from that food will help rehydrate the brown sugar.
Barring all of these great ideas, you can also make a brown sugar substitute by combining one cup of white sugar with three tablespoons of molasses. There's also pourable brown sugar available at most supermarkets, which is all but impossible to—though it does lack some of that stickiness that is most often associated with the not-as-easily poured stuff.
Really, when a recipe calls for brown sugar, it's usually best to use brown sugar and take the time to revive your rock hard block of sucrose. And once your once-hardened brown sugar is back in action, remember to store it properly so that you don't have to go through this rigamarole all over again. You can get a reusable airtight container on Amazon for less than $10, and it's worth it, because having soft brown sugar ready to go whenever you need it is priceless.