Roxanne's Millionaire's Shortbread

Our answer

Normally a grainy caramel is caused by sugar not being fully dissolved before the carmel is boiled, the sugar then re-crystallizes as it cools and causes a grainy texture. However in the shortbread recipe the sugar should have alredy been dissolved fully in the condensed milk and furthermore golden syrup is added which usually helps to reduce the risk of cryastallization.

In the absence of any problems with the ingredients we would guess that the caramel mixture may have become slightly overheated so it may be worth stirring every 30 seconds rather than every minute in the last 3 minutes of cooking (once the whole mixture has heated through and is just starting to bubble). Also make sure that your spoon and bowl don't have any traces of sugar on them as once crystals are introduced to a boiling sugar mixture they can act as a "magnet", pulling already dissolved sugar crystals towards them and causing the mixture around them to crystallize too.

If you are having a persistent problem then you could switch to a ready-made product. Many UK supermarkets now sell caramelized condensed milk which is sold in the UK as Nestle Caramel (it is usually in cans next to the condensed milk). The caramel is also known as dulche de leche and can be found in jars in delicatessens and some supermarkets in other countries. You may need to warm the caramel gently so that it is spreadable. You will need about 600g (approx. 1 1/2 cups) of this to cover the shortbread.