Cream the butter and sugar in the bowl of an electric mixer, or use a large bowl with a hand-held electric beater. Beat in the vanilla and egg, then add the flour. Beat until smooth, remove from the bowl, halve and shape into discs. Wrap with clingfilm and chill for about 40 minutes until firm. Heat oven to 180/fan 160C/gas 4.
Dust the work top with flour and roll out one portion of dough to 3mm thick. Cut stars or other shapes with cutters.
Line 2 baking sheets with non-stick baking paper and lift the cookies onto it using a palette knife. Chill for 10 minutes and then bake for 10-12 minutes until just turning golden at the edges. Cool on a baking rack. When cool, brush lightly with egg white then sprinkle with silver balls or sugar crystals.