Whisk together flour, cocoa, salt and cinnamon; set aside.
Using an electric mixer, cream butter and sugar until light, about 2 min. Beat in eggs and vanilla. Working in two batches, add flour mixture and beat until just combined. Divide dough in half and wrap pieces in plastic wrap, flatten into disks and chill 1 hr or overnight.
Heat oven to 350ºF. On a floured surface roll dough to ¼ inch thick. Cut with a 5-inch gingerbread man cookie cutter and transfer to unlined baking sheets. Reroll scraps once. Bake until cookies spring back when touched, about 12 min. Cool on wire racks.
Prepare 1 recipe Royal Icing. With an electric mixer mix confectioners' sugar, meringue powder and water on low speed, scraping sides of bow,l until icing is the consistency of soft whipped cream, about 10 min. Transfer to a piping bag fitted with a #2 plain tip. Decorate with lines for a skeleton. Pipe an outline of a skull and circles for eyes and mouth; flood face with icing. Sprinkle some with sanding sugar. Dry completely.
Chocolate skeleton cookies are nothing but good old bones! Sweet icing bones and dark chocolate cookies, that is.