2 x 20cm (8in) round sandwich tins, buttered and base lined
Method
Set the oven to Gas Mark 5 or 190°C.
To make the sponge, put the eggs, butter, brown sugar, golden syrup, melted chocolate and vanilla extract into a large bowl.
Sift in the flour, cocoa and baking powder and add the water.
Beat with a hand-held electric mixer until smooth.
Divide the mixture between the two tins and smooth the tops level.
Bake in the centre of the oven for 25-30 minutes, until the cakes feel springy to the touch. Remove the cakes from the oven and leave them to cool in the tins for a few minutes before turning them onto a wire rack to cool completely.
To make the frosting,
Stir the soured cream or crème fraiche into the melted chocolate using a wire whisk.
Allow the mixture to cool slightly but not to set.
Place one layer of sponge on a plate and spread a generous layer of frosting over it and then place the other sponge on top.
Cover the cake completely with the remaining chocolate frosting, swirling it on.
Leave the cake in a cool place for the frosting to set a little before serving.