You will also need: 18cm (7in) square brownie tin, lined with baking parchment
Mix the crushed biscuits with the melted butter and then lightly press the mixture into the tin to make an even layer. Chill for 20 minutes.
Place the butter and sugar into a non-stick pan and stir over a medium heat until the butter melts and the sugar dissolves. To make a golden caramel, 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 and then cool and chill for about half an hour. Pour the melted chocolate over the caramel, smoothing to the edges.
When the chocolate has hardened a little, cut into squares.