1 To make the cinder toffee grease and line a shallow baking tray. Gently heat the caster sugar and golden syrup in a large heavy-based pan until all the sugar has dissolved. Turn up the heat and rapidly boil – do not stir the mixture, just swirl the pan if it starts to go darker on one side.
2 Keep boiling until the mixture turns a lovely golden brown colour – it should take approximately 5 minutes. Remove from the heat. Add 1½ teaspoons of the bicarbonate of soda and stir for a few seconds, being very careful as the mixture will bubble, fizz and expand quite a bit. Tip the mixture into the baking tray and leave to set.
3 Preheat the oven to 180C/Gas Mark 4. Grease and line two 23cm cake tins.
4 Put the prunes in a bowl and cover with the hot water. Soak for 20 minutes or so, then blend until smooth.
5 Sift the flour and 1 teaspoon of bicarbonate of soda into a bowl. In a separate bowl, beat the butter and dark and light brown sugars until pale and fluffy. Add the egg a little at a time, adding a spoonful of flour between additions to prevent any curdling. Fold in the remaining flour, add the prune mixture and vanilla extract and stir thoroughly.
6 Pour into the tins and bake for 35-40 minutes or until a skewer inserted in the centre of the cakes comes out clean. Remove from the oven and leave to cool in the tins.