I'm making sugar cookies and I have ran out of granulated sugar. I only have confectioner's sugar and dark brown sugar left.
Can I substitute one of these for white granulated sugar? Which would be the better substitute?
I've done this before, and you won't get exactly the same cookie as you got before.
Confectioner's sugar is a total bomb. Don't bother.
Dark brown sugar makes darker cookies, with a chewier texture, which keeps longer.
This isn't always a bad thing. Several of my cookie recipes I thought were a bit dry were saved by DBS. It added a nice depth to the flavor, as well.
The creaming step (beating sugar into fat) of cookie making creates air bubbles in the dough which will expand during baking. Powdered sugar (confectioner's sugar) won't create these bubbles, which is why it doesn't make a very good substitute for white sugar.
Brown sugar is a more moist than white sugar, and will result in chewier cookies. Due to the molasses component, it also has an effect on flavor - not necessarily a bad effect, but definitely noticeable. It will also make a darker-colored cookie, so be extra careful when evaluating your cookies for done-ness in the oven.
For me, a trip to the neighbor's (or the grocery store, if I must) is always worthwhile.