1.Preheat oven to 180ºC. Grease a 20cm x 30cm (8-inch x 12-inch) rectangular slice pan; line base and long sides with baking paper, extending paper 5cm above sides.
2.Place coconut in a small ovenproof dish; brown, in oven, stirring frequently, for 2 minutes or until coconut is lightly golden, cool.
3.Stir butter and dark chocolate in a medium saucepan over low heat until smooth. Cool for 10 minutes.
4.Stir sugar and egg into chocolate mixture, then stir in sifted flour and sour cream. Spread mixture into pan.
5.Bake about 35 minutes; cool brownie in pan.
6.Melt white chocolate in a medium heatproof bowl over a medium saucepan of simmering water (don’t let water touch base of bowl).
7.Chop marshmallows coarsely; combine with nuts and coconut in a medium bowl, then sprinkle over brownie slice. Drizzle with white chocolate. Refrigerate to set before cutting.