Caramel Shortbread

from an English cookbook with British measurements, so get out your kitchen scale!

4 oz butter, softened
2 oz sugar
6 oz self-raising flour
Pinch of salt

TOPPING:
4 oz superfine sugar
4 oz butter
2 Tablespoons golden syrup (*You cannot buy golden syrup in supermarkets. You have to order online or go to World Market in Tucson. There is really not a good substitute for this unique syrup; you can use corn syrup...it works, but it doesn't give the same rich flavor as golden syrup)
218 gm tin condensed milk (= 7.657 oz)
100 gm bar plain or milk chocolate (=3.512 oz) (*I used more chocolate than this so I could cover the whole thing with a thicker layer)

Set oven to 325F. Well grease and base line an approx. 11 x 7 inches shallow tin. Cream the butter and sugar together in a bowl until light and fluffy. Stir in the flour and salt and knead well. Put into the tin, spread out and press down evenly with the fingers. Bake for 30 minutes until pale brown. Leave in the tin to cool. Meanwhile, for the topping, put all ingredients, except the chocolate, into a pan. Bring to the boil and continue boiling for 5 minutes or more, stirring continuously until the mixture thickens and turns pale caramel brown. When ready, pour over the cooked shortbread and spread out. When set, melt the chocolate in a bowl set over hot water, pour over the caramel and spread out. When the chocolate has set, cut into squares with a sharp knife dipped in hot water.