Nigella Lawson's Sour Cream Chocolate Cake

This is my 7th cake in the chocolate cake search and was recommended by some of you bloggers!  It comes from Nigella Lawson, so that in itself was promising.  She described that the sour cream provides a "wonderful mouth-filling smoothness".  She was right (like always) as the texture of the cake was velvety smooth and the icing was equally luxurious.  I actually declared that it was the best chocolate icing I've ever tasted.  This is a favourite cake and now one of the top contenders! The only thing that lets it down, it that the smooth texture hardens slightly in the fridge which you may see from the image.  I would therefore recommend serving it at room temperature.

Ingredients
200g plain flour
200g caster sugar
3/4 tsp baking powder
1/4 tsp bicarbonate of soda
1/2 tsp salt
200g soft unsalted butter
40g best cocoa
150mL sour cream
2 lge eggs
1 1/2 tsp vanilla extract

Ingredients for Icing
80g milk chocolate
80g dark chocolate
75g unsalted butter
125mL sour cream
1 tsp vanilla extract
1 tbsp golden syrup
300g icing sugar, seived
1/2 tsp hot water

Method
Preheat the oven to 180 degrees (fan-forced) and prepare 2 sandwich cake tins.  Combine flour, sugar, baking powder, bicarb and salt in large bowl.  Using electric mixer, add butter.  In a measuring jug, whisk together cocoa, sour cream, eggs, and vanilla, then slowly add this mixture to flour ingredients in bowl; beating until thoroughly mixed.  Pour batter into tins and bake for 30 minutes; when they're ready cakes should be starting to shrink back from edge of tins.  Leave for 10 minutes in tins on racks, then turn out to cool.
To make icing, melt chocolate and butter.  Let cool a little, then stir in sour cream, vanilla and syrup.  Add icing sugar, and a little hot water, blending until smooth.  Place one cake on plate and ice the top; leaving a slight 1cm border.  Place the other cake on top and ice this too.  Then go around the sides of the cakes to ice.  There will be enough icing to generously ice the middle and top of the cakes; so don't be scant on one certain level.