The best caramel shortbread, or millionaires shortbread, in the world can be found at the Boston Tea Party coffee shop in Exeter, UK. They have a few other branches in the South West of England.
Update: Top quality caramel shortbread should be made from quality chocolate, caramel and shortbread. Many places try to cut corners by using a crushed biscuit base rather than actual crumbly shortbread. Shortbread is best made with top quality white flour, caster sugar, butter (not margarine or spread), and a pinch of salt. Once the ingredients are fully mixed, put the mixture in a baking tray but do not press down – leave it loose.
Caramel can be made perfectly adequately by boiling an unopened can of condensed milk in water for two hours. Simply put the whole can unopened in a saucepan and submerge it in water. Bring to the boil and allow to simmer for two hours.
Once that is done, simply spread the caramel mixture over the cooked and cooled shortbread and the melted chocolate over that. Allow to cool before eating.
How to melt chocolate… well I’m sure you can work that out!
Here’s an alternative, but still quite simple recipe that gives another way of making the caramel.