Powdered sugar — also called confectioners' or icing sugar — is ordinary white sugar ground to an extremely fine, flour-like powder and blended with a little starch to keep it free-flowing. It dissolves almost instantly, dusts as a soft white veil, and whips smoothly into icings and glazes, which is why it is the backbone of buttercream, royal icing, glaze and the snowfall over doughnuts and beignets rather than a sugar you would cream into batter.