Chocolate hazelnut sour cream cake

Ingredients

• 200 gram dark cooking chocolate, chopped
• 1 teaspoon bicarbonate of soda
• 1 cup (250ml) boiling water
• 250 gram butter, at room temperature
• 1 1/4 cup (275g) caster sugar
• 3 eggs, separated
• 1 teaspoon vanilla extract
• 2 1/2 cup (375g) plain flour
• 1 teaspoon baking powder
• 2/3 cup (160g) light sour cream
• 1/2 cup (75g) toasted hazelnuts, chopped
• pure icing sugar, to dust
• whipped cream, to serve (optional)