Grease deep 19cm-square cake pan. Fold 40cm piece of foil lengthways into thirds, place foil strip over base and up two sides of pan (this will help lift the slice out of the pan). Line base with baking paper.

Stir butter, sugar and milk in medium heavy-based saucepan over medium heat, without boiling, until sugar dissolves. Bring to the boil, boil, stirring constantly, about 10 minutes or until caramel mixture becomes a dark-honey colour and starts to come away from the base and side of pan.

Working quickly and carefully (the mixture is very hot), pour caramel into pan, smooth with metal spatula. Press nuts into caramel with spatula, cool 20 minutes.

Stir chocolate and extra butter in small heatproof bowl over small saucepan of simmering water until smooth, spread mixture over slice. Refrigerate until set.

Use foil strip to lift slice from pan before cutting into squares.