In a medium bowl, beat together the butter and sugar until light and creamy. Beat in the egg yolk and vanilla extract.
Add the flour and salt. Stir until it forms a dough, then knead until smooth. Wrap in clingfilm and chill for 30 minutes.
Preheat the oven to 180°C, fan 160°C, gas mark 4. Roll out the dough on a floured surface to the thickness of a £1 coin.
Stamp out shapes with cutters and bake on a non-stick baking tray for about 10 minutes, until lightly browned. (An adult should help remove the hot baking tray from the oven.)
Cool on a wire rack. Place the icing sugar in a bowl and add cold water, a little at a time, until it's a thick pouring consistency. Spread a little on each cookie and decorate with sprinkles while the icing is wet and leave to dry out.
Cook's tip: Try swapping 25g of the flour for cocoa powder to make chocolate biscuits. For the icing, swap 3 tablespoons of the icing sugar for cocoa powder and mix with warm water.
You can buy different shaped cookie cutters from our Sainsbury's baking range.