Marble cake

I have been making marble cake from Peyton and Byrne's recipe book.  In googling the recipe I was surprised to find my blog as the fourth result, and thought I should probably add the recipe. I always used to make the Baker and Spice recipe but this one has the edge
So here it is the official one, http://www.hellomagazine.com/cuisine/recipes/201104185283/cake/chocolate/recipe/
and here is my minimum washing up quick method,
125g unsalted butter(or omit the salt and use salted)
170g sr flour
1/2 tsp bicarbonate of soda
1/2 tsp salt
200g caster sugar
3 large eggs
1tsp vanilla essence
150g plain yoghurt
40g cocoa powder
60ml boiling water
I used my kitchen aid to cream together the butter and sugar, I then added the eggs alternately with the flour, bicarb and salt.  Then mixing in the yoghurt and vanilla.  In the bowl you have been weighing the ingredients in mix the cocoa with the boiling water.  Then add half the cake batter to the cocoa mix  (Not as the 10 yr old son did and added it all to create a light chocolate cake that didn't win the yummiest cake competition).  Grease and line a loaf tin and dollop alternate spoons of the mixes.  Finally zig zag a line through the mix with a pointed knife to create the ultimate marble effect.  Cook in the oven (180c or bottom right of a four door aga) for 40 minutes.  The proper recipe has a white chocolate icing but ours have always got eaten before I have iced it.