Cream the butter and sugar. Add the egg and beat until fluffy. Blend in the vanilla. Sift in the flour, baking powder, and salt, mixing until just combined. Wrap the dough in plastic wrap and chill for 1 hour.
Preheat the oven to 350 degrees. On a floured surface, roll half of the dough to 1/8-inch thick. Use a ghost cookie cutter, or you can use a small knife to cut the dough into freeform ghost shapes, as shown. Gather and reroll scraps. Continue with remaining dough. Place the cookies on a greased baking sheet and bake for about 8 minutes, until just turning golden. Do not allow cookies to brown.
When the cookies are completely cool, cover them with white royal icing and press chocolate chips into the icing to make eyes (and a mouth if desired). Let the icing dry before stacking the cookies.
With beaters or a standing mixer, whip the egg white and lemon juice until frothy. At medium speed, beat in the confectioners' sugar, a little at a time, until the mixture is thick but still liquid enough to beat. Then beat on high until the mixture is thick and glossy, about 3 minutes. Cover the surface with plastic wrap while waiting to use it. Royal icing will set to a firm, glossy finish when applied to a cookie. The icing can be stored, tightly covered, in the refrigerator for up to a week.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.