Preheat oven to 350F, putting in a baking sheet as you do, and line a loaf tin (21x11cm) with baking parchment.
Put the flour, baking soda, cocoa, sugar, butter, eggs, vanilla and sour cream into the food processor and blitz till a smooth, satiny brown batter. Scrape down with a spatula and process again while pouring the boiling water down the funnel. Switch it off, remove lid and blade, then stir in the chocolate chips.
Scrape and pour this beautiful batter into the prepared loaf tin and slide into the oven, cooking for about an hour. When it's ready, the loaf will be risen and split down the middle -- but your cake skewer may come out a bit sticky, as this is a very damp cake -- this is a good thing.
When the cake has had about 45 minutes of baking, put the syrup ingredients of cocoa, water, and sugar into a small saucepan and boil hard for 5 minutes or more, until it reduces into a dark, smokey, chocolatey syrup.
Take cake out of oven and set on cooling rack and still in it's pan, pierce here and there with a skewer. Then pour the syrup as evenly as possible, which is not very, over the surface of the cake. It will run to the sides of the pan, but some will absorb in the middle.
Once the cake is completely cold, remove from pan and remove the parchment. Sit on oblong plate. Now take bar of chocolate and cut with a very sharp knife so that it splinters and flakes and falls in slices of varying thickness and thinness. Scatter these pieces all over the surface of the cake.