Preheat oven to 180 degrees C. Not necessary to grease pan.
In a sauce pan combine 1 1/4 cup water and 65g butter. Heat until butter melts then stir in brown sugar and coconut. Pour into ungreased 20x30cm pan. Sprinkle marshmallows and nuts over top. Set aside.
For the cake: in a saucepan over low heat combine chocolate with 1/2 cup (125ml) water. Heat, stirring, until chocolate is melted. Remove from heat.
In a large bowl mix flour, sugar, bicarb soda and salt. Add sour cream, 125g butter, vanilla and eggs. Add chocolate mixture and beat 3 minutes. Carefully spoon batter over coconut marshmallow mixture in pan.
Bake for 40 to 50 minutes, or until toothpick inserted into centre comes out clean. When cool, invert on a plate.