Grease a 4.5cm-deep, 20cm (base size) square cake pan. Line base and sides with baking paper, allowing a 2cm overhang on all sides.
Process biscuits until finely chopped. Add melted butter. Process until combined. Press mixture over base of prepared pan. Cover. Place in the fridge for 30 minutes to set.
Meanwhile, place condensed milk, sugar and extra chopped butter in a saucepan over medium-low heat. Cook, stirring, for 10 minutes or until sugar has dissolved. Increase heat to medium. Cook, stirring constantly, for 10 minutes or until mixture turns golden and thickens.
Pour the caramel over prepared base. Set aside for 10 minutes. Cover. Place in the fridge for 2 hours or until set. Cut and serve.
