Millionaire shortbread

This week I've made millionaire shortbread.  Different from normal shortbread as it has caramel topped by milk chocolate on top of the shortbread.  It is very rich and delicious.
I have attempted it several times in the past, with the most memorable being the one where the caramel set rock hard and had to be chipped out of the dish!  The last two times I've used the recipe from the tin of condensed milk and the caramel has been perfect.