Preheat the oven to 180°C (160°C Fan, Gas Mark 4). Line 2 large baking trays with baking parchment. In a bowl, beat the butter and sugar together until soft and creamy. Add the flour and stir into the creamed butter and sugar until the mixture forms a firm dough.
Turn on to a lightly floured surface and knead gently until smooth. Roll out to a thickness of ½ cm 1/4 inch) - the mixture will be quite short. Using a 2cm (3/4 inch) fluted edged round cutter, stamp out 100 shapes, re-rolling as necessary.
Arrange on the baking trays, spaced slightly apart. Gently push down on the top of each with your finger to flatten the tops and chill for 30 minutes.
Bake for 8-10 minutes until lightly golden. Cool for 5 minutes then transfer to a wire rack to cool completely.
To decorate, sift the icing sugar into a bowl. Make up the Egg White Powder as directed on pack and whisk until very stiff. Stir in the Madagascan Vanilla Extract.
Gradually add the sifted icing sugar to the Egg White Powder in small batches, whisking well after each addition, to form a thick, peaking icing.
Divide the icing into as many portions as colours you want to make. Add a few drops of a different Gel Food Colour to each portion, and mix in thoroughly to make different coloured icing.
Put the icing into piping bags fitted with a large closed star nozzle. Holding a shortbread round between your forefinger and thumb, pipe a star on top. The icing will be firm and probably rise to a peak as you pull the nozzle away. Gently tap the biscuit on the work surface to distribute the bottom of piped star evenly over the surface of the biscuit.
Leave the icing to set for at least 30 minutes to firm on the outside before serving.