Place butter and brown sugar in bowl of stand mixer fitted with paddle attachment. Mix on low until combined then increase speed to medium. Mix until light and fluffy.
Turn off mixer and scrape sides of bowl. Add molasses and mix until well incorporated. Turn off mixer and scrape sides of bowl again.
Whisk flour, salt, and spices together. Add to wet mixture. Then add cold water and mix on low to combine.
Remove dough from bowl and form into 2 flat disks. Wrap in plastic wrap and refrigerate until firm.
Preheat oven to 350 degrees.
Place first disk onto lightly floured surface and roll until ¼-inch thick. Cut into desired shapes. Place on parchment-lined sheet pan and bake until done, about 15 to 20 minutes. Cool completely before icing with Royal Icing.
Chef's Note:Royal icing is the best choice for assembling and decorating gingerbread houses. Most other icings contain shortening or butter that will eventually soak into the gingerbread and could cause your house to soften and collapse. And this icing works well on cookies too.
Place all ingredients in bowl of stand mixer fitted with paddle attachment. Beat on low until powdered sugar is absorbed, then increase speed to high and beat until snow-white and fluffy, about 3 to 5 minutes.
Remove from bowl and keep covered with damp towel to retain moisture and avoid hardening.