Preheat the oven to Gas Mark 2, 150°C, fan 130°C. Put the egg whites in a large mixing bowl and beat with an electric mixer or hand whisk until white and frothy. Gradually (to stop it from curdling) add the sugar, whisking continuously until the mixture is thick, glossy and holds stiff peaks.
Using a spatula, fold in cornflour and vinegar the until completelty incorportated.
Grease and line a 20cm x 28cm (8in x 11in) Swiss roll tin. Spoon the mixture into the prepared tin and smooth the surface, taking care not to knock the air out of the meringue. Bake for 30 minutes, or until crisp to the touch and just turning golden.
While the meringue is cooking, make the compote. Put the blueberries in a pan over a low heat and add the icing sugar. Cook gently for 5 minutes, mashing with a fork until the sugar has dissolved and the berries have softened and released their juice. Remove from the heat and pass the berries through a sieve into a bowl. Set aside to cool.
Remove the meringue from the oven and cover with a clean, damp tea towel for 15 minutes. This prevents it from becoming overly crisp.
Meanwhile, whip the cream with an electric whisk, adding the sugar gradually until combined and thick. Stir in the vanilla extract. Set aside.
Dust a large piece of nonstick baking paper with icing sugar. Carefully turn out the cooked meringue onto the sugared paper and gently peel off the paper from cooking. Evenly spread the cream over the meringue, leaving 2cm (1in) border around the edges. Scatter with the raspberries, strawberries and blueberries. Drizzle over the compote.
Beginning with the short end, tightly roll up the meringue, using the paper to help you. Transfer to a serving plate, dust with icing sugar and decorate with any remaining fruit. Serve with a spoonful of compote.
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