Cream together the softened butter and sugar in a large bowl until light and fluffy. Add the egg and vanilla extract, beating until fully combined.

In a separate bowl, whisk together all-purpose flour and cocoa powder. Gradually add to the wet mixture, stirring until a soft dough forms. Cover and refrigerate for at least 1 hour.

Preheat the oven to 350°F. Roll the chilled dough out on a floured surface to 1/4-inch thickness. Cut into cat shapes and place on a parchment-lined baking sheet.

Bake for 9 to 10 minutes until just set but still soft. Allow to cool completely on a wire rack.

Once cool, cover each cookie with black royal icing and let set for about 20 minutes. Pipe green icing for eyes on each cookie.

Serve as festive Halloween treats. Store in an airtight container at room temperature for up to one week.