Preheat oven to 180°c/160°c fan-forced. Line 2 baking trays with baking paper.
Using electric beaters, beat butter and sugars together until pale and creamy, then beat in egg until combined.
Sift flours and cocoa over butter mixture, then fold in using a spatula. Stir in dark chocolate.
Roll dough into a log about 5cm thick and 23cm long. Wrap in plastic wrap, then refrigerate for 20 minutes or until firm.
Cut log into 1.5cm-thick slices, then place 2cm apart on trays, then bake for 15 minutes or until edges are firm. Cool on a wire rack.
Using the writing icing, pipe a dot in centres of chocolate bits to make eyes.
Divide icing between 2 small bowls, then combine blue and red to create purple in one bowl and combine red and yellow to create orange in second bowl. Spread frosting onto half of the cookies. Sandwich with remaining cookies. Coat sides in sprinkles, place eyes on cookies and serve.