Preheat oven to 150degC.
Cream together the butter, sugar and salt until pale and fluffy. Sift the flour and beat into the creamed ingredients to make a rough dough. Divide the dough into four pieces and and flatten each one into a patty shape about 4cm thick. Wrap in plastic wrap and refrigerate 1 -2 hours or until firm. Remove from fridge one at a time and roll each piece out between two sheets of baking paper to about 5mm (1/4inch) thickness. 
Use Christmas cookie cutters (Christmas trees, angels, bells, stars, gingerbread men) to cut out shapes. Transfer shapes carefully, using a palette knife, to trays lined with baking paper. Repeat with remaining dough. 
Bake cookies for 20 minutes, then turn down heat to 100degC (200degF) and bake for 10-15 minutes or until lightly browned.
Remove from oven and transfer to cake racks to cool. Decorate when cold.
Fit a piping bag with a nozzle with a fine point and half fill the bag with white frosting.  Pipe decorative patterns over cookies and decorate with cachous and coloured sugars.
For the White Frosting: Sift the icing sugar. Whisk the egg whites until stiff in a mixing bowl and gradually add the icing sugar, lemon juice and glycerine (if using).  Keep beating until thick. Cover tightly with plastic wrap until ready to use, or put into a plastic container with tight fitting lid (the frosting dries out quickly when exposed to air).