1. Preheat oven to 350°.
2. To prepare cookies, weigh or lightly spoon flour into dry measuring cups, and level with a knife. Combine flour, 1/2 teaspoon salt, baking powder, ginger, cinnamon, cloves, and allspice, stirring with a whisk. Place butter and granulated sugar in a large bowl, and beat with a mixer at high speed until light and fluffy. Add egg, and beat until well blended. Beat in molasses and vanilla extract. Reduce mixer speed to low. Add the flour mixture to butter mixture; beat just until combined.
3. Shape dough into a 4-inch round, and cover with plastic wrap. Chill for 1 hour. Roll dough to 1/4-inch thickness on a lightly floured surface. Cut out 40 (2 x 3-inch) cookies, rerolling scraps as necessary. Place cookies 1 inch apart on a baking sheet lined with parchment paper. Bake at 350° for 9 minutes or until lightly browned on bottoms. Cool on a wire rack.
4. To prepare icing, combine powdered sugar, egg white powder, and 1/8 teaspoon salt, stirring well. Gradually add 1 tablespoon water, stirring constantly with a whisk until smooth. Decorate cookies as desired. Sprinkle with sugar, if desired. Let the cookies stand on a cooling rack until icing is completely dry (about 1 hour).
Finishing flourishes don't require special equipment. First, “flood” cookies by spreading thin frosting with a small offset spatula to coat, if desired. Pipe thicker frosting from a zip-top bag to outline. Use a toothpick to make small dots of icing that act as glue for decorative details like sugar pearls.