Diana’s fudgy chocolate cake - Stephanie Alexander

Ingredients

• 125 g butter, chopped into 5 cm pieces
• 100 g dark chocolate, roughly chopped
• 1 cup boiling water
• 1 cup castor sugar
• 1 teaspoon pure vanilla
• 2 eggs, separated
• ½ cup sour cream
• 1 teaspoon bicarbonate of soda
• 2 cups self-raising flour
• pinch of salt