Method
Prep:20min  ›  Cook:4hr20min  ›  Extra time:3hr25min  ›  Ready in:8hr5min 

1. For the caramel, remove the label from the condensed milk tin. Place the unopened tin in a deep pot and fill with water until the water reaches about 10-12cm above the top of the tin. Bring the water to the boil and boil steadily for 3-4 hours. ALWAYS MAKE SURE THE TIN IS TOTALLY COVERED WITH WATER! If the water level gets too low, the tin will explode.
2. While the caramel is boiling, make the shortbread. Preheat the oven to 180 C / Gas mark 4. Cream the butter and sugar together and add the vanilla. Beat until smooth. Add flour and salt and press onto the bottom of a greased 23cm square tin. Bake 20-25 minutes until shortbread is pale golden. Set aside and allow to cool.
3. After boiling the caramel, remove the tin with tongs and place in a bowl of lukewarm water to cool. When the tin is cool enough to handle, open it carefully and pour over the shortbread. Allow to set in the refrigerator for at least 20 minutes.
4. When the caramel has set, melt the chocolate in the microwave with the butter and spread it in an even layer over the caramel.
5. Leave the bars out at room temperature to harden a little. Before the chocolate has completely set, cut into squares.