Originally a semi-refined sugar that still had some of the natural molasses left in it, brown sugar is now typically made by spraying white sugar with molasses. Brown sugar makes cookies soft and chewy. It helps cakes and pastries stay moist. And it gives a warm spiciness and a hint of caramel essence that its white counterpart can't offer. Light brown sugar contains less molasses (about 3-1/2%) than dark brown (6-1/2%), accounting for differences in color and flavor. In general, if a recipe only calls for "brown sugar," it refers to light brown. But dark brown and light brown are virtually interchangeable in recipes, though dark will give you a deeper, spicier flavor.
1 lb. = 2-1/4 cups packed
In a pinch, make your own: To make 1 cup brown sugar, combine 1 cup white granulated sugar with 3 to 4 Tbs. molasses in a food processor. Pulse to blend. Muscovado sugar is also a fine substitute (though it's harder to find than regular brown sugar).
Remember to press brown sugar into the cup when measuring. Simply scooping it up will give you a lot of air along with the sugar, and an inconsistent measurement. Sometimes bits of brown sugar harden into small nibs. Unless you're dissolving or melting the sugar, press it through a sieve to get rid of the lumps so they don't create pockets of crunchy sugar bits in your finished dish.
Store brown sugar tightly wrapped in a cool, dark place. If you find that your sugar has dried out and hardened, sprinkle it with a few drops of water and microwave it on low for 15 to 20 seconds. Heating the sugar gently like this will soon bring it back to its original soft state. If you don't have a microwave, tuck a slice of sandwich bread in with the sugar to soften it overnight.