the necessary cake

Baked the chocolate cake again on Saturday, for Richard and B's joint birthday party. Lots of people asked me for the recipe, so I'll post it here (again):
The cake: 400g flour, 250g caster, 100g light muscovado, 50g cocoa, 2tsp baking powder, 1tsp bicarb, 3 eggs, 150ml sour cream, 1 Tbsp vanilla, 200g butter, 100ml veg oil, 300ml water. Two 7" round cake tins. Preheat to 180C. Soften butter, then whizz all ingredients together. Bake until a tester comes out clean (about 1 hour, but I've seen a lot of variation).
The icing: 175g dark chocolate, 250g butter, 275g icing sugar, 1 Tbsp vanilla. Melt choc and butter together, sift sugar, whizz.
The cream: Some cream, whipped.
Wait for the cakes to cool, apply icing and cream in layers. Serve.
My whole approach to weights and measures is a bit slapdash, which doesn't seem to do this recipe any harm.