1. Preheat the oven to 150°C/fan130°C/gas 2. Lightly oil and line a rectangular tin, measuring about 26.5 x 17 x 3.5cm deep with baking paper - make sure the paper is 3cm higher than the tin sides to form a collar.
2. In a large, grease-free bowl, whisk the egg whites to soft peaks. While whisking, slowly add the sugar, 1 tablespoon at a time, until you have a thick meringue. Whisk in the vinegar and cornflour. Spoon evenly into the tin (it will be very full), and level the surface. Bake for 45 minutes, or until the meringue is slightly risen and a very pale golden. Set aside to cool completely in the tin.
3. Meanwhile, make the coulis. Put the strawberries, icing sugar and Cointreau into a food processor. Whizz until smooth, then pass through a nylon sieve into a bowl. Set aside, or cover and chill until needed.
4. Whisk the cream and Cointreau to soft peaks - it will continue to thicken as you spread it, so don't overwhip!
5. Turn the pavlova out onto a large board. Wet a large, sharp knife and use it to trim the pavlova edges, re-wetting the knife as you go to prevent the meringue sticking to it. Spread the cream evenly on top of the pavlova.
6. Cut the pavlova into 8 squares and put on serving plates. Top each with some berries and dust with icing sugar. Drizzle the strawberry coulis around the plates to serve.
Tip: Make but don't decorate the pavlova the day before. Cool, store in a tin, covered, in a cool place overnight. The coulis will also keep overnight in the fridge. Complete the recipe from step 4 to serve.