1.Grease deep 23cm (9 inch) square cake pan; line base and sides with baking paper, extending paper 5cm over sides.
2.Process biscuits until fine; add melted butter, process until combined. Press mixture over base of pan. Refrigerate about 20 minutes or until firm.
3.Stir dark chocolate and chopped butter in small saucepan over low heat until smooth. Remove from heat; stir in peanuts and coconut, spread mixture over base. Refrigerate about 30 minutes or until firm.
4.Combine white chocolate and marshmallows in medium bowl; spread marshmallow mixture over nut layer. Refrigerate about 1 hour or until firm.