01 - Whisk together flour, cocoa powder, cinnamon, baking powder, and salt in a medium bowl until well combined.
02 - Beat butter and sugar in a large bowl until light and fluffy, approximately 2 minutes. Add egg and vanilla, mixing until fully incorporated.
03 - Gradually add dry ingredients to wet mixture, mixing just until dough forms. Avoid overmixing to prevent tough cookies.
04 - Divide dough in half and flatten into disks. Wrap in plastic and refrigerate for at least 30 minutes until firm.
05 - Preheat oven to 350°F. Line baking sheets with parchment paper.
06 - Roll dough to 1/4-inch thickness on lightly floured surface. Cut out skeleton shapes using cookie cutter.
07 - Place cookies on prepared baking sheets. Bake for 10-12 minutes until set. Cool completely on wire rack.
08 - Beat egg white or meringue powder with powdered sugar and water until smooth and pipeable consistency is achieved.
09 - Transfer icing to piping bag with fine tip. Pipe skeleton designs onto cooled cookies. Allow icing to dry completely before serving.