In a saucepan set over low heat, or in the microwave, melt butter, then stir in the brown sugar, molasses, salt, and spices.
Transfer the mixture to a medium-sized mixing bowl, let it cool to lukewarm, and beat in the egg.
Whisk the baking powder and soda into the flour, and then stir these dry ingredients into the molasses mixture.
Divide the dough in half, and pat each half into a thick rectangle. Wrap well, and refrigerate for 1 hour or longer. The dough may be sticky and hard to roll if not thoroughly chilled, so make sure it's cold before continuing.
Preheat your oven to 350°F. Get out several baking sheets; there's no need to grease them, though lining with parchment saves effort on cleanup.
Once the dough has chilled, take one piece of dough out of the refrigerator, and flour a clean work surface. Roll the dough 1/8" to 1/4" thick; the thinner you roll the dough, the crispier the cookies will be. Flour both the top and bottom of the dough if it starts to stick. Alternatively, place the dough on parchment, and put a sheet of plastic wrap over it as you roll, pulling the plastic to eliminate wrinkles as necessary when rolling; this will keep dough from sticking without the need for additional flour.
Cut out shapes with a floured gingerbread man cookie cutter, cutting them as close to one another as possible to minimize waste.
Transfer the cookies to ungreased cookie sheets (or, if you've rolled right onto the parchment, remove the dough scraps between the cookies). Place a snowflake icing decoration in the center and fold the arms over the snowflake. Bake the cookies just until they're slightly brown around the edges 8 to 12 minutes, or until they feel firm. Let the cookies cool on the baking sheets for several minutes, or until they're set. Transfer them to a rack to cool completely. Repeat with the remaining dough.
For the royal icing: Beat all ingredients together until icing forms peaks (7-10 minutes at low speed with a heavy-duty mixer, 10-12 minutes at high speed with a hand-held mixer). If the mixture is too thick, thin out with a bit of water. Decorate the cookies with royal icing.