Make a sponge cake. with a usual recip.
Stone and chop lots of plums, (about 500g before stoning for an 8" tin) and mix them in with the sponge mix. Take a few remaining plums and halve and stone them, them lay them flay side down in the bottom of the cake tin. Pour mixture on top of halved plums and bake as normal.
Meanwhile take another 500g of stoned chopped plums and make a syrup with some sugar and a little water. Cook till the plums break down and make a jam like substance.
When cooled slice the sponge in half and spread the plum jam mix in the middle.
When cooked invert cake so cut plums are uppermost. Delicious served warm with vanilla ice cream or extra thick double cream. Needs to be kept in an airthight container and eaten in 3 to 4 days.