melt 1oz butter gently in a pan and add finely crushed shortbread biscuits, stirring constantly till the butter has soaked in. Remove from heat and lightly press the mixture into a 8" tin lined with baking parchment to make an even layer.
Chill for 20 minutes.
Whilst base is chilling, Place the butter and sugar into a non-stick pan and stir over a medium heat until the butter melts and the sugar dissolves. Add the condensed milk, stirring continuously, until the first bubbles appear on the surface. Remove from the heat as soon as it comes to the boil.
Spread the caramel evenly over the crumb base.
Chill for about half an hour.
Break chocolate into a heatproof bowl and gently stir over a pan of simmering water till melted.
Pour the melted chocolate over the caramel, smoothing to the edges.
When the chocolate has hardened a little, cut into squares.
Make it healthier
You can halve the caramel filling without compromising on taste. I usually make the full batch of caramel but divide it into two and double up on the other ingredients to make two trays.