Method

1. To make the cake: preheat the oven to 190°C (375°F, Gas 5).
2. Grease and flour two 21.5cm/8.5 inch sandwich tins and line the bottom of each with a round of non-stick baking paper.
3. Sift the flour and bicarbonate of soda into a bowl, add the caster and light muscovado sugar and make a well in the centre.
4. Pour the water into a small saucepan, add the cocoa powder and butter and stir over a moderate heat until the butter melts and the mixture becomes smooth – do not allow to boil.
5. Remove from the heat and add the vanilla extract.
6. Stir the vinegar into the cream and pour into the flour.
7. Add the cocoa mixture and eggs and whisk everything together until smooth.
8. Divide the mixture equally between the prepared tins.
9. Bake for 20-25 minutes or until the cakes have risen and are springy and firm to the touch.
10. Remove from the oven, loosen the sides with a knife and turn onto wire racks to cool.
11. To make the filling: put the chocolate and water into a small saucepan and stir over a low heat until melted and smooth, then set aside.
12. Put the caster sugar, cream and butter into another saucepan and stir constantly over a moderate heat for 4-5 minutes until it becomes smooth and thick, taking care not to let it burn.
13. Remove from the heat and stir in the chocolate and vanilla.
14. Allow to cool, stirring occasionally, then chill until cold.
15. Sandwich the two cakes together with the filling and leave on a rack over a tray.
16. To make the icing: put the chocolate into small saucepan with the butter and water.
17. Stir over a low heat until the chocolate melts and the mixture becomes smooth and will coat the back of the spoon thickly, but do not overheat.
18. Pour the icing over the top of the cake, allowing it to trickle randomly down the sides, then chill until set.
19. This cake is best made a day before you need it as it improves in flavour.
20. Serve with whipped cream.