Take whatever you need out of the fridge so that all ingredients can come to room temperature.
Preheat the oven to gas mark 3/170ºC, putting in a baking sheet as you do so, and line a 900g loaf tin (approx. 21x11cm and 7.5cm deep) with clingfilm – it won’t melt!
Put the flour, bicarb, cocoa, sugar, butter, eggs, vanilla and sour cream into a food processor and blitz till a smooth, satiny brown batter. Scrape down with a rubber spatula and process again while pouring boiling water down the funnel. Switch it off then remove the lid and the well-scraped double-bladed knife and, still using your rubber spatula, stir in the chocolate chips or morsels.
Scrape and pour this beautiful batter into the prepared loaf tin and slide into the oven, cooking for about 1 hour. When it’s ready, the loaf will be risen and split down the middle and a cake-tester, or a fine skewer, will pretty well come out clean. But this is a damp cake so don’t be alarmed at a bit of stickiness in evidence; rather, greet it.
Not long before the cake is due out of the oven – say then it’s had about 45-50 minutes – put the syrup ingredients of cocoa, water and sugar into a small saucepan and boil for about 5 minutes. You may find it needs a bit longer: what you want is a reduced liquid, that’s to say a syrup, though you can take it further to caramelize the sugar.
Take the cake out of the oven and sit it on a cooling rack and still, in its tin, pierce here and there with a cake tester. Then pour the syrup as evenly as possible over the surface of the cake.
Let the cake become completely cold and then slip out of its tin, removing the clingfilm as you do so. Sit on an oblong or other plate Grate over chocolate flakes to finish.