Preheat oven to 180°C. Line the base and sides of a 18cm square tin with baking paper, allowing paper to overhang the sides.
Spread macadamia nuts evenly over a baking tray and cook in preheated oven for 7 minutes or until toasted.
Place the biscuits in the bowl of a food processor and process until finely crushed. Transfer to a medium bowl and add the melted butter. Stir until well combined. Press the biscuit mixture evenly over the base of the lined pan and smooth the surface with the back of a spoon. Place in the fridge for 1 hour to chill.
Place condensed milk, golden syrup and 30g of the unsalted butter in a medium non-stick saucepan. Cook over medium heat, stirring constantly, for 10-15 minutes or until the mixture thickens and turns a caramel colour. Remove from heat and add the toasted macadamia nuts. Stir until well combined. Spoon the hot caramel over the biscuit base and smooth the surface. Place in the fridge for 1 hour to set.
Place the chocolate and remaining butter in a small clean, dry heatproof bowl. Place the bowl over a small saucepan of barely simmering water over low heat (make sure the bowl doesn’t touch the water). Use a dry metal spoon to stir gently for 2 minutes or until mixture melts and is smooth. Pour the chocolate mixture evenly over caramel filling and smooth surface with a knife. Place in the fridge for 15 minutes or until the chocolate sets. Cut into 12 pieces to serve.