Grease a 6cm- deep, 9cm x 19cm (base) loaf pan. Line base and sides with baking paper, allowing a 2cm overhang at long ends. Combine Tim Tam, hazelnuts, macadamias and raisins in a bowl.
Place chocolate, condensed milk and butter in a saucepan over low heat. Cook, stirring, for 3-4 minutes or until smooth. Add chocolate mixture to biscuit mixture. Stir to combine.
Spoon mixture into prepared pan, pressing down on mixture to prevent air bubbles. Smooth surface with a spatula. Cover. Refrigerate for 2 hours or until set. Invert cake onto a plate. Remove baking paper. Dust with cocoa. Slice. Serve.