We just added the blackberries to an ordinary sponge cake recipe and then we did one cake in the mould and made fairy cakes with the rest. We made the filling with whipped cream, about a tablespoon or so of pureed and sieved blackberries and a tablespoon of sugar. This is an old whisk from my childhood. The boys really like using it so I'm glad I rescued it!
I always leave some cakes without icing/cream as James and Charlie are much more likely to eat them plain - we let them choose. Charlie ate 4 cakes!! They were very nice!