Bring all ingredients to room temperature.
Pre-heat your oven to 170C and stick a baking sheet in. Grease a loaf tin (21 x 11cm and 7.5cm deep) and line it with parchment paper. Leave a little bit of parchment to extend beyond the tin so that you will have something with which you can easily lift the cake out of the tin later on.
Put the flour, baking soda, cocoa, sugar, butter, eggs, vanilla, and sour cream in a processor and blitz until it is smooth and satiny. Scrape down with a rubber spatula and process again while pouring the hot water down the funnel. Switch it off, remove the lid and the blades, and then fold in the chocolate chips.
Pour the batter into the prepared tin and bake in your pre-heated oven for 1 hour or until a cake tester comes out pretty clean...there may be stickiness as this is a damp cake and that is totally fine.
When you still have about 10-15 minutes left of baking time on your cake, get started with the syrup. Put the cocoa, water, and sugar in a saucepan and boil for 5 minutes or so. You want a reduced, syrupy liquid that is still pourable.
When the cake is done, place the tin on a rack and pierce the cake here and there with a skewer. Pour the syrup as evenly as you can over the cake.
Once the cake is completely cooled, take it out of its tin (just pull it out with the parchment overhang you so carefully crafted), peel off the parchment, and place it on a serving platter. Now take your bar of chocolate and cut it into flakes and splinters and scatter all over the top of the cake. I used milk chocolate for the topping because the cake was already dark chocolate all throughout and I was taking this to a friend who is more a fan of milk than dark. You can also try topping with chocolate before the cake cools down so the chocolate shards melt a bit..