Beat together the butter and sugar in a bowl until smooth and creamy. Remove one cooled cake from its case and take a thick, angled slice off the top. Spread a little of the buttercream over another cake, position the slice on top and spread with a little more buttercream to make a larger 'face' cake.
Spread a large, rectangular board with 4 tablespoons of the buttercream and scatter with the chocolate. Spread the remaining buttercream over the cakes, then position in a snaking line over the chocolate with the face cake at the front.
Roll out the coloured icing thinly on a work surface lightly dusted with icing sugar and cut out 9 rounds using a 5 cm (2 inch) cookie cutter. Place on all the cakes except the face and end cakes. Cut out a pointed tail from the trimmings and place on the end cake. Roll out the chocolate icing and cut out a slightly larger round. Position on the front cake. Thinly roll out the chocolate icing and cut out 2.5 cm (1 inch) rounds. Place on the rest of the cakes and top each with a candy-covered chocolate sweet. Shape and position eyes and mouth using icing trimmings and the two remaining sweets.