There are times when it's very nice to have the Internet handy--to be able to search quickly for something you really need to know, like how to "fudge" an extra cup of powdered sugar for a Treehouse dweller's chocolate birthday cake icing.
On the other hand, it's important to double-check your sources. The first "substitutes" site I found via Google suggested blending one cup of granulated sugar and 1 cup of cornstarch in the food processor. Thinking that sounded a little odd, I checked another site, which had an almost identical chart: 1 cup of granulated sugar--oh, 1 tablespoon of cornstarch. Well, that made more sense. Two more sites--both said 1 tablespoon. The first one must have been just a typo.
But boy, I hope nobody tries that 1 cup-1 cup recipe. Yech. One little word makes a big difference!
[Update: if you try this, it seems to take an awful lot of processing before you end up with anything close to powdered sugar--it's more like superfine. I hate to admit it, but we ended up using a can of icing from the corner store instead, after monkeying with that icing for awhile and still crunching sugar. What did we do with the first icing? Made it into cookie balls and sent them to work with Mr. Fixit (for his co-workers). There, now you know ALL.]