baking with icing sugar vs granulated sugar

Probably. Commercial icing or powdered sugar contains a small amount of cornstarch to act as an anticaking agent. Plus because it is ground so finely, it takes about twice as much powdered sugar to replace the granulated stuff. So if your recipe called for 1 cup of sugar, you would need about 1 3/4 cup of powdered sugar. Since you likely had less sugar and a bit of cornstarch, your cookies would bake up paler as a result. They probably are not quite as sweet tasting either. Not sure why you are trying to use up the icing sugar, but you would probably do better to add it as a sweetener to smoothies or other drinks. It doesn't go bad, really, so you can also store it for a long time in a simple zip lock type baggie in the cupboard.