In the bowl of the stand mixer, cream the butter, adding the sugar gradually. Cream the mixture until it is light.
Add the beaten eggs and beat the mixture thoroughly at medium speed.
Sift 3 cups of flour with the salt, cinnamon, and nutmeg, and add it alternating with the sherry. The mixture will be very wet.
Add more flour, 1 to 2 cups, to stiffen the dough, though it should not be very stiff, just enough to roll it out and hold its shape.
Divide the dough into four pieces and press into disks. Wrap each disk in plastic and chill for several hours.
Preheat the oven to 450° F/230° C.
Between sheets of plastic wrap, roll the chilled dough very thin, about 1/16 – ⅛ inch (~1.5 – 3 mm) and cut it out with cookie cutters in star, diamond, or heart shapes.
Keep the dough on the plastic wrap and lay a piece of parchment on top of it, gently pressing onto the cut dough. Flip it over onto the baking pan, leaving the plastic on top. Chill the dough for about 5 minutes or more depending on the temperature of your home and how soft the dough is. If it’s too soft, you might have difficulty peeling away the excess dough.
Once it has chilled, carefully peel away the plastic and dough trimmings leaving the cookies on the parchment.
Bake in the preheated oven for about 7 minutes, watching carefully to make sure the edges don’t burn. Remove from the pan and allow to cool completely on wire racks.
In the bowl of the stand mixer, add the sifted confectioners sugar and the egg white powder, stirring to mix.
In a measuring cup, stir together the water, lemon juice, and vanilla. (If you want to color the icing, add some liquid coloring at this point. Reduce the amount of water slightly to compensate for the extra liquid.) Pour the mixture into the sugar and mix on low until all of the dry ingredients are incorporated, about 1 minute.
Increase mixer speed to high and whip for 3 minutes until the icing has stiff peaks.
Spoon into prepared piping bags and decorate as desired. Allow the cookies to rest uncovered for an hour or so to make sure the icing hardens and becomes stiff. Wrap cookies individually for gifts.