1) Cream together the butter, confectioners' sugar, and corn syrup until light and fluffy.
2) Beat in the eggnog flavor, nutmeg, and salt.
3) Add the baker's ammonia or baking powder to the egg and water, and stir to dissolve.
4) Add this mixture, along with the flour, to the ingredients in the bowl, and beat until smooth.
5) Divide the dough in three pieces, wrap them in plastic wrap, and refrigerate for 1 hour, or overnight.
6) If the dough has been in the fridge for 1 hour, give it about 20 to 25 minutes to warm up a bit. If it's been refrigerated overnight, let it rest at room temperature, covered, for about an hour and 15 minutes. While the dough is resting, preheat your oven to 350°F.
7) Roll the dough 1/8" thick on a lightly floured work surface (a silicone rolling mat works well here).
8) Cut out shapes, and transfer to ungreased or parchment-lined cookie sheets.
9) If desired, whisk 1 egg white with 1 tablespoon water, paint the cookies with this egg wash, and sprinkle with colored sugar.
10) Bake the cookies just until they're slightly brown on the edges, or until they feel firm, about 8 to 10 minutes.
11) Let the cookies cool on the baking sheet for several minutes, or until they're set. Transfer them to a rack to cool completely. Cookies may be frosted after baking, if desired.