- Take everything out of the fridge so that all the ingredients can come to room temperature.
- Preheat the oven to 180°C and line and butter two 20 cm sandwich tins with removable bases.
- Put all the cake ingredients into a food processor and process until you have a smooth, thick batter.
- If you want to go the long way around, just mix the flour, sugar and leavening agents in a large bowl and beat in the soft butter until you have a combined and creamy mixture. Now whisk together the cocoa, sour cream, vanilla and eggs and beat this into your bowl of mixture.
- Divide this batter, using a rubber spatula to help you scrape and spread, into the prepared tins and bake until a cake tester comes out clean, which should be about 35 minutes, but it is wise to start checking at 25. Also, it might make sense to switch the two cakes around in the oven halfway through cooking time.
- Remove the cakes, in their tins, to a wire rack and let cool for 10 minutes before turning out of their tins. Don’t worry about any cracks as they will easily be covered by the icing later.
- To make this icing, melt the butter and chocolate in a good-sized bowl either in the microwave or suspended over a pan of simmering water. Go slowly either way, you don’t want any burning or seizing.
- While the chocolate and butter are cooling a little, sieve the icing sugar into another bowl. Or, easier still, put the icing sugar into the food processor and blitz. This is by far and away the least tedious way of removing lumps.
- Add the golden syrup to the cooled chocolate mixture, followed by the sour cream and vanilla and then when all this is combined whisk in the sieved icing sugar. Or just pour this mixture down the funnel of the food processor on to the icing sugar, with the motor running.
- When you’re done, you may need to add a little boiling water – say a teaspoon or so – or indeed some more icing sugar: it depends on whether you need the icing to be runnier or thicker; or indeed it may be right as it is. It should be liquid enough to coat easily, but thick enough not to drip off.
- Choose your cake stand or plate and cut out four strips of baking parchment to form a square outline on it (this stops the icing running on to the plate). Then sit one of the cakes, uppermost (ie slightly domed) side down.
- Spoon about a third of the icing on to the centre of the cake and spread with a knife or spatula until you cover the top of it evenly. Sit the other cake on top, normal way up, pressing gently to sandwich the two together.
- Spoon another third of the icing on to the top of the cake and spread it in a swirly, textured way (though you can go for a smooth finish if you prefer, and have the patience). Spread the sides of the cake with the remaining icing and leave a few minutes till set, then carefully pull away the paper strips.
The second cake was a bit higher than the first one. I had some purple flowers that were the same as Nigella used in her show, so I put them on the nicer-looking cake.
The icing is delicious, but mainly because I like Sacher torte, I once spread apricot marmalade between the two layers of the cake and covered it with Nigella’s icing. It was good too, but it tastes better with just the regular icing. It’s very sweet, but I usually don’t add the entire amount of powdered sugar in it (300 grams). I just keep adding it and taste it every once in a while. When I can’t taste the sour cream anymore, I stop adding the sugar. This is a very sweet cake with a strong chocolate flavor and you can’t really eat more than one tiny piece at a time. Of course, I eat a lot of those tiny pieces so the cake doesn’t last longer than two days. Ever. It’s one of the best cakes I’ve ever tasted.