Brown sugar is simply granulated sugar with molasses added back, which gives it moisture, a soft texture, and a caramel-toffee flavor. Light brown sugar has less molasses (milder); dark brown sugar has more (richer). They’re usually interchangeable, with dark lending a deeper flavor where you want it.
Why it behaves differently
The molasses makes brown sugar moist and slightly acidic, which keeps cookies chewy and soft and reacts with baking soda to give lift. It’s measured packed — pressed firmly into the cup — because its moisture makes it clump; a packed cup weighs about 220 grams.
Fixing hard brown sugar
Brown sugar hardens when its moisture escapes. Soften it by sealing it with a slice of bread or a damp paper towel for a day, or warm it briefly in the microwave to make it workable again right before use.
Frequently asked questions
Light or dark brown sugar? Interchangeable in most recipes; dark has more molasses and deeper flavor.
Why measure brown sugar packed? Its moisture makes it clump, so packing gives a consistent amount.
How do I soften hardened brown sugar? Seal it with a slice of bread or damp towel overnight.