Grease and line a 16cm x 26cm slice pan with baking paper, allowing the 2 long sides to overhang. Place the dark and milk chocolate in a heatproof bowl over a saucepan of simmering water (make sure the bowl doesn't touch the water). Melt the chocolate, stirring occasionally, until smooth.
Combine the Turkish delight, biscuit, pistachio and marshmallow in a bowl. Pour over melted chocolate and stir to combine. Spoon into prepared pan. Place in the fridge for 2 hours or until set. Cut into pieces.