In a large bowl, sift together cocoa powder, flour, baking soda, and salt. In another bowl, cream the butter and sugar until fluffy. Add egg and vanilla, mixing well.

Gradually blend dry ingredients into wet mixture until a soft dough forms. Divide dough, wrap in plastic wrap, and refrigerate for at least 2 hours.

Preheat the oven to 350°F (175°C). Roll out the dough to ¼-inch thickness and cut out shapes using cookie cutters. Place on a parchment-lined baking sheet.

Bake for 12 minutes or until edges set. Cool on a wire rack for 10 minutes.

Once cooled, decorate cookies with royal or glaze icing. Let set for at least 30 minutes before serving.