Beat the butter and sugar on medium until light and fluffy. In a large measuring cup combine the baking soda and 1/4 cup hot water. Stir until dissolved then add the molasses.
In a medium mixing bowl combine the flour, ginger, cinnamon, salt and allspice. Add the flour mixture to the butter mixture alternately with the molasses mixture, beginning and ending with the flour. Shape the dough into a ball and cover and chill at least 1 hour or overnight.
Preheat oven to 350° F. Roll the dough out to ¼-inch thick on a lightly floured work surface. Cut dough into desired shapes. Place the cookies 2 inches apart on a parchment lined baking sheet. Bake for 15 minutes. Allow the cookies to cool on the baking sheet for 2 minutes then transfer to a wire rack to cool completely.
Spoon Royal Icing into a piping bag fit with a small tip and pipe icing onto the cooled cookies. Add candies, crystals or colored sugar as desired. Allow the icing to dry completely. Store cookies in an airtight container.