If you are out of powdered sugar, put regular sugar in the blender and process on high until it turns powdery. Be sure to stir up the sugar on the bottom a couple of times so it all turns out to be the same consistency. Measure out the amount needed for your recipe!
You add corn starch to keep the sugar from clumping and caking. Commercial manufacturers of powedered sugar do this, which I learned of when in college Food Prep. courses.
This is a page about how to make confectioners (powdered) sugar. You don't need to run to the store to buy powdered sugar for the recipe you are making; make your own.