Crowd-pleaser chocolate cake

Name: Crowd-pleaser chocolate cake
Cook time: 2 h
Total time: 2 h
Cuisine: Australian
Ingredients: 200 gram dark chocolate, chopped finely; 1/3 cup (35g) cocoa powder; 1/2 teaspoon teaspoon bicarbonate of soda; 1 1/3 cup (330ml) boiling water; 250 gram butter, softened; 1 3/4 cup (385g) firmly packed brown sugar; 2 teaspoon vanilla extract; 4 eggs; 1 cup (150g) plain flour; 2 cup (300g) self-raising flour; 2/3 cup (160ml) sour cream, at room temperature; fresh flowers, to decorate, optional; 375 gram dark chocolate, melted; 3/4 cup (180ml) sour cream; 1 1/2 cup (240g) pure icing sugar, sifted