Preheat the oven to 120C (fan).
Soften the butter - the easiest way to do this is in a microwave on full power for 20 second bursts, stirring each time. The butter should be very soft and smooth but not melted.
Cream the softened butter together with the caster sugar until pale in colour. This can be done by hand with a wooden spoon or in an electric mixer on high speed. It should only take a few minutes if the butter is soft enough.
Add the flours and mix to combine. If using an electric mixer mix on minimum speed.
Divide the mixture equally in three and place each part in one of the sandwich tins. Spread the mixture and press down evenly then mark around the outside with a fork and prick all over to prevent the shortbread rising unevenly.
Bake for approx 1 hour 30 minutes until pale golden (it should not be brown) and firm to touch if pressed in the centre. Remove from the oven and immediately cut into 12 equal sized pieces. Allow to cool in the tin.
When cooled, sprinkle with extra caster sugar and tap the sides of the tin until the pieces are released.