In a mixing bowl, use a hand-held mixer to beat the butter until light and fluffy. Add the sugar and beat into the butter until well combined.
Add the egg and the vanilla and beat into the butter mixture.
Next, add the flour, cocoa powder, and baking powder, and with the mixer on low speed, blend into the butter mixture just until the flour is incorporated.
Divide the cookie dough in half and wrap each half in plastic wrap and place in the refrigerator for 1 hour.
Preheat oven to 350 degrees. Prepare a baking sheet by lining it with parchment paper. Set aside.
Remove the cookie dough from fridge and roll dough a 1/4 inch thick between two sheets of parchment paper. Cut out desired shapes.
Transfer cookies to prepared baking sheet. Bake for 12 minutes. Remove from oven and allow to cool for a full 10 minutes of the baking sheet before transferring to wire cooling rack. DO NOT REMOVE THE COOKIES FROM THE BAKING SHEET UNTIL A FULL 10 MINUTES HAS PASSED.
Roll out the rest of the dough and start again. Repeat until all dough has been used.
When the cookies are completely cooled, prepare the royal icing by beating all ingredients together until the icing is smooth and glossy.
Transfer icing to a piping bag fitted with a writing tip and slowly outline the shape of the cookie.
Allow the icing to harden before stacking. Can be left on the counter top in a food safe container for 5-7 days.