In a large bowl, cream butter and sugars until light and fluffy. Beat in egg and vanilla. Combine the flour, cocoa and baking soda; gradually add to creamed mixture and mix well. Cover and refrigerate for 1-2 hours or until easy to handle.
On a lightly floured surface, roll dough to 1/8-in. thickness. Cut with a floured 3-in. gingerbread boy cookie cutter. Place on greased baking sheets.
Bake at 375° for 7-8 minutes or until set. Cool for 1 minute before removing from pans to wire racks to cool completely.
For icing, in a small bowl, combine confectioners' sugar and milk until smooth. Cut a small hole in the corner of a resealable plastic bag; fill with icing. Pipe skeleton bones on cookies.