Sift flour with salt, then cream the butter and caster sugar together until pale and creamy. With the motor running, gradually add the egg; if the mixture looks like it's curdling then add a tbsp of sifted flour.
Put the mixer on it's lowest setting and add the flour gradually. Stop mixing as the mixture comes together.
Roll out onto a floured surface; about 5mm thick. Cut out biscuits using the metal cutter ring. Now bake for about 20-30 minutes until lightly brown in colour.
Let the biscuits cool on a wire rack for 5 minutes.