For the pavlova:
1) Preheat the oven to 80°C/gas mark 1.
2) Place a sheet of baking paper on a baking tray. Draw a 23-cm circle on the paper, using a 23-cm plate as a guide, then turn the paper over so the circle is on the reverse side. (This way you won't get a pencil mark on the meringue.)
3) Place the egg whites and salt in the bowl of an electric mixer fitted with a whisk attachment. Beat the egg whites on high speed until firm, about 1 minute. With the mixer still on high, slowly add the sugar and beat until it makes firm, shiny peaks, about 2 minutes.
4) Remove the bowl from the mixer, sift the cornflour onto the beaten egg whites, add the vinegar and vanilla, and fold in lightly with a rubber spatula. Pile the meringue into the middle of the circle on the baking paper and smooth it within the circle, making a rough disk.
5) Bake for 1 1/2 hours. Turn off the oven, keep the door closed, and allow the meringue to cool completely in the oven, about 1 hour. It will be crisp on the outside and soft on the inside.
6) Invert the meringue disk onto a plate and spread the top completely with sweetened whipped cream. Combine the strawberries, blueberries and raspberries in a bowl and toss with about 120ml of raspberry sauce, or enough to coat the berries lightly. Spoon the berries carefully into the middle of the pavlova, leaving a border of cream and meringue.
7) Serve immediately in large scoops with extra raspberry sauce.
For the whipped cream:
1) Whip the cream in the bowl of an electric mixer fitted with a whisk attachment (you can also use a hand mixer). When it starts to thicken, add the sugar and vanilla and continue to beat until firm. Don't overbeat.
For the raspberry sauce:
1) Place the raspberries, sugar and 60ml water in a small saucepan. Bring to a boil, lower the heat, and simmer for 4 minutes.
2) Pour the cooked raspberries, the jam, and framboise into the bowl of a food processor fitted with a steel blade and process until smooth. Chill.