Beat butter and sugar at medium speed with an electric mixer until fluffy. Stir together baking soda and 1/4 cup water in a small bowl until dissolved; stir in molasses.
Stir together flour and next 4 ingredients. Add to butter mixture alternately with molasses mixture, beginning and ending with flour mixture. Beat at low speed after each addition. Divide dough in half; flatten each into a disk. Wrap disks in plastic wrap, and chill 1 hour.
Preheat oven to 350°. Roll dough disks to 1/4-inch thickness on a lightly floured surface. Cut dough into desired shapes, rerolling remaining dough scraps as needed. Place cookies on parchment paper-lined baking sheets.
Bake at 350° for 12 to 15 minutes. Cool in pans on wire racks 5 minutes; remove from pans to wire racks, and cool completely (about 30 minutes).
To assemble cookies into a gingerbread house or luminary, spoon a small amount of Royal Icing into a zip-top plastic freezer bag. Snip 1 corner of bag to make a small hole, and pipe icing onto edges of cookies. Fit edges together, holding sides in place 5 to 10 minutes or until dry enough to adhere.