Start by laying a base row of sugar cubes around a 7-inch-diameter cardboard circle, leaving space for the entrance. Using a mortar of 2 egg whites mixed with 3 cups confectioners' sugar, add subsequent layers of cubes, one row at a time, decreasing the circumference gradually as you go. Apply the mortar to the top cubes, not to those already in place. Be sure to work alternately left and right from the entrance toward the back of the igloo and stagger the cubes the way a builder lays bricks. Build a total of 10 layers, stopping halfway through the construction to let the igloo dry. Make the arch and roof separately, working on a flat surface. When they are dry, glue them in place. Allow the igloo to dry completely, then sprinkle with a blizzard of sugar.