Preheat oven to 325 degrees F. Beat shortening and sugar until light. Add molasses, egg and vanilla and beat until smooth. In another bowl, sift dry ingredients. Gradually stir dry ingredients into molasses mixture. Turn dough unto lightly-floured surface and knead into ball. Place dough on a good quality, non-stick or parchment paper-lined 11 x 17-inch baking sheet. With floured hands press and shape dough into a rectangle that's about an inch thick. With a lightly floured rolling pin, roll gingerbread until it fills the pan. You can press and pull the gingerbread with your fingers to make it exactly fit. Poke gingerbread all over with a fork. Bake in oven 25 minutes.
Cool gingerbread on rack for 2 hours. While the gingerbread cools, you can draw and cut out cardboard templates of the different pieces needed to build your house. You will need 1 gingerbread base to build the house on that's 4 1/4 inches wide, and 7 inches wide, 2 sides of the house that are 2 inches wide, and 4 1/2 inches long, two roof pieces that are 4 inches wide, and 6 inches long, and 2 pieces for the front and back of the house that are 4 1/4 inches long along the bottom, 2 inches high to where the slope of roof begins, and 5 inches tall to the centre of the roof.
Carefully slide gingerbread out of the pan on to large cutting board or work surface. Place templates on top of gingerbread as shown in the photograph. With a sharp knife carefully cut out the pieces to the house. Cut out, a cookie cutter works well, a door in the front piece, and windows in the side pieces. Place cut out pieces back on to the baking sheet.
(Reserve reaming gingerbread for creating a chimney, a fence, or logs for the house). Bake in a 325 degrees F oven 10 minutes more. Remove from oven and let pieces stand, uncovered, overnight. (This will dry out the gingerbread and make it sturdy enough for a house).
Make icing by whipping egg whites until frothy. Gradually mix in icing sugar until icing is thick enough to form peaks. (Icing dries rapidly, keep covered at all times. A piping bag with a plain tip is a good tool to use when applying icing).
Ice together the frame of the house, allowing it to set before placing on roof. Place on roof, hold in place for a few minutes to prevent it from sliding.
Set house on decorative tray. Decorate as you like with icing, Christmas candies, pretzels, and cereal. For a snowy effect, dust house with icing sugar after completing.