Pre-heat the oven 325F / 165C degrees.

Sift the flour into a bowl .

Grease a 7 inch square baking tray or 8 inch circular cake tin if you are making petticoat tails.

Put butter into a bowl and add the pinch of salt if using unsalted butter.

Mash butter with a fork until it is soft and creamy and all lumps have gone.

Add sugar and mix quickly and incorporate well.

Add the flour a little at a time - use your fork at the beginning and then knead well on a lightly floured board.Knead for about 7-10 minutes.

Press the kneaded shortbread into your prepared tin. (1/2 in thick).

Prick surface with a fork and bake in oven for 35-45 minutes, making sure not to let it get too brown.

Whilst still quite warm in the tray, mark across and cut into required shapes. but leave them there in the tray until cooled.

Sprinkle with castor sugar.