I was going to make Peanut Butter Cookies as a treat today, but found that I don't have any brown sugar.  Can I just substitute plain white sugar instead?

-Paula-

Yes, you can substitute plain white sugar in place of the brown sugar, but you will find that the texture of the finished product will be slightly different and that the cookies will likely remain crisper.  Brown sugar is hygroscopic, meaning that it will absorb moisture from the air.  Over time cookies made with brown sugar become softer.

Alternatively, if you have molasses on hand, you can make the equivalent of brown sugar by using white sugar and molasses.  In fact, modern brown sugar is simply that -- white sugar which is blended with varying amounts of molasses.

To substitute for one cup of light brown sugar, use one cup of granulated sugar plus one or two tablespoons of molasses.  To substitute for one cup dark brown sugar, use a cup of granulated sugar plus two or three tablespoons of molasses.  The exact amount of molasses will depend in part on whether you are using a light or dark molasses.  Light molasses is sometimes called fancy molasses.

The sugar and molasses can simply be added to the recipe without needing to premix them.