I found a Passover recipe for making confectioner's sugar with potato
starch instead of cornstarch. (Recipe from Penina W. Freedenberg from
some other mailing list or web page.) I'm not sure if potato starch
and cornstarch work the same way but I'd recommend using her
proportions and see how things work out.
Anyway, the recipe said to
Take 1 cup minus 1/2 Tbsp sugar
1/2 Tbsp potato starch
Process in a food processor until very fine powder.