1. Grease a 4cm-deep, 19cm (base) square cake pan. Line base and sides with baking paper, allowing a 3cm overhang on all sides.
2. Process biscuits until mixture resembles fine breadcrumbs. Add melted butter. Process to combine. Using the back of a metal spoon, press mixture over base of prepared pan. Refrigerate for 15 minutes. Arrange hazelnuts over biscuit mixture.
3. Make filling Place chopped butter, sugar and milk in a heavy-based, non-stick saucepan over medium-low heat. Cook, stirring constantly (as mixture can stick to pan), for 10 to 12 minutes or until mixture thickens and becomes golden. Pour over hazelnuts. Using the back of a spoon, smooth top. Refrigerate for 10 minutes.
4. Meanwhile, place chocolate and cream in a small microwave-safe bowl. Microwave on High (100%) for 30 seconds to 1 minute, stirring with a metal spoon, or until smooth.
5. Spread chocolate mixture over filling. Cover and refrigerate for 3 hours or until set. Cut into pieces. Serve.