Place the butter in a large mixing bowl with the caster sugar. Using a wooden spoon beat until light and fluffy. Gradually beat in the egg until well mixed, then gradually add the flour. Use your hands to bring the mixture together to a stiff dough. Gather into a ball and chill for 30 minutes.
Heat the oven to Gas 4, 180°C, fan160°C. Roll out the dough on a work surface lightly dusted with flour until it's the thickness of a £1 coin. Using an oval cutter (or a cardboard egg-shaped template), cut out egg-shaped biscuits, using rolled out trimmings to make more. Place on baking trays and bake in the oven for 6-10 minutes, depending on size, until just golden brown.
Allow to cool for 5 minutes before removing from the baking tray to cool completely.
To decorate, make up the icing as directed on the packet. Divide between 3-4 bowls adding a little food colouring to each bowl. Place the mixture into piping bags and decorate biscuits as liked - kids may like to use sprinkles or sweets too.
These biscuits will keep for between one and two weeks when stored in a sealed cake tin.
Freezing and defrosting guidelines
Bake according to the recipe but stop before adding decoration or icing if called for. Allow to cool completely. Wrap well in clingfilm and tin foil (to prevent freezer burn) and freeze for up to 3 months.
Loosen the tin foil and clingfilm but leave to defrost in packaging at room temperature for several hours before serving. If you wish to serve the baked goods warm, place them on a baking sheet for 10 minutes in a moderate oven.
If you wish to add icing or decoration, add them after the baked goods have fully defrosted and are at room temperature.