Using an electric mixer, cream butter and sugar until light and fluffy. Add molasses and egg and beat well. Dissolve baking soda in milk, then add that to the batter also.
Sift together the dry ingredients, then add to the batter, mixing thoroughly. Wrap the dough and chill for at least 24 hours and up to 1 week.
ON BAKING DAY
Heat oven to 350˚F. On a very lightly floured surface, roll out the dough thinly – not quite as thin as you’d roll a pie crust – and cut into shapes. Place the cookies on parchment-paper lined baking sheet leaving one inch between them; they will not spread much.
Bake at 350˚ 6 to 8 minutes, until the cookies are firm but not much browned. Transfer the parchment with cookies to a rack to cool.
There are several options for decorating.
Before baking: mix 1 tablespoon cold water with the white of one egg, and brush this on the cookies. You might also choose to press nuts or candied fruits on the tops.
After baking: drizzle with a simple confectioners sugar and milk glaze, or as shown above, use a more substantial royal icing and apply various cookie decors while the icing is still wet. This will harden in an hour or two, making the cookies easy to pack.