In the bowl of a standing or electric mixer, cream butter and sugar until light and fluffy. Add molasses and egg and beat well. Dissolve soda in milk and add to batter. Sift together ginger, cinnamon, salt, and flour, and add to batter, mixing thoroughly. Chill dough at least 24 hours.
Adjust oven rack to middle position and heat to 350 degrees. Roll dough out to ⅛-inch thickness and cut into gingerbread boy shapes. Whip egg white with a tablespoon of cold water and use a soft-bristled pastry brush to glaze cutouts, giving cookies a shiny surface and covering imperfections. Press candied fruit or nuts on cookies for eyes or noses, as desired.
Arrange cookies on a baking sheet and bake 6 to 8 minutes or longer, until firm but not too brown. Cool on racks until room temperature. Frost and decorate as you like.
In the bowl of a standing or electric mixer, cream butter and egg yolk until smooth. Stir in confectioners’ sugar and flavoring. Add some food coloring if desired.