To prepare the base
Preheat oven to 220″C. Position oven shelf near the bottom of the oven.
Draw a 22 cm circle onto a sheet of baking paper and place face down onto a baking tray.
Using a large mixing bowl, Beat egg whites and cream of tarter on a medium speed until light and foamy.
Add cornflour and vinegar and continue to beat until soft peaks form. Gradually add sugar, 1 tbsp at a time, while beating on high speed until thick and glossy
Fold through vanilla. Spoon mixture onto paper to cover circle Using a palette knife, smooth side and top.
Place into oven and reduce heat to 120 “C. Cook for 1 hour 15 minutes.
Turn off oven and prop open oven door with a towel or wooden spoon.Allow the pavlova to cool down slowly for 2 to 3 hours or overnight preferably.
To make plum blackberry compote
Place the plums, cut side up, in a small roasting tin. Sprinkle with the sugar, add the lemon zest and juice. Bake for 15 minutes at 180 “C, then remove and turn over the plums. Scatter the blackberries over the top. Bake for a further 5 minutes until the plums and blackberries have softened nicely. The compote is ready to serve. Bring it to the room temperature before topping it up on the pavlova.
To assemble the pavlova
Set the pavlova on a platter and spoon the double cream over it. Using a slotted spoon, arrange the plum blackberry compote on the cream; reserve the juices in the bowl.
Cut the Pavlova into wedges and transfer to plates. Drizzle some of the reserved juices around each wedge and serve immediately.