In a saucepan, combine 300ml water and 50g butter. Heat until butter melts, then stir in dark brown soft sugar and coconut. Pour into ungreased 20x30cm tin. Sprinkle marshmallows and walnuts over top. Set aside.
In a saucepan over low heat, combine chocolate with 125ml water. Heat, stirring, until chocolate is melted. Remove from heat.
In a large bowl, mix flour, sugar, bicarbonate of soda and salt. Add soured cream, 115g butter, vanilla and eggs. Add chocolate mixture and beat for 3 minutes. Carefully spoon batter into baking tin.
Bake at 180 C / Gas 4 for 40 to 50 minutes or until skewer inserted into centre comes out clean. Place tin on foil onto baking tray to prevent messy spillages.