Spread the buttercream in a thin layer over the tops of the cooled cakes using a small palette knife.
Knead the icings on a work surface lightly dusted with icing sugar. Take 50 g (2 oz) of the white icing, roll out thinly and cut out 4 rounds using a 6 cm (2½ inch) cookie cutter. Cut out 6 small stars from each round using a cutter. Roll out a little of the blue icing thinly and cut out 24 stars. Fit the blue stars into each white round and transfer to 4 of the cakes.
Roll out another 50 g (2 oz) of the white icing thinly. Roll tiny balls of blue icing between your finger and thumb. Press at intervals on to the white icing. Gently roll with a rolling pin so that the blue icing forms dots. Cut out 4 rounds and transfer to 4 more cakes.
Cut long strips 5 mm (¼ inch) wide from the remaining blue and white icing and lay them together on the work surface. Roll lightly with a rolling pin to flatten and secure them together, then cut out 4 more rounds. Place on top of the remaining 4 cakes.