From a cookbook, I will copy this. For semi-sweet cholocate: 6 tablespoons cocoa plus 7 tablespoons sugar plus 1/4 cup shortening equals one 6 ounce or 1 cup chocolate chips or 6 1 ounce blocks semi-sweet chocolate. I am not familiar with icing sugar but assume it must be like powdered sugar. If it looks similar I would use it. May take just a little less or more. As you are mixing it, when it looks the way you want it to, go for it. Hope it turns out okay.