Heat the oven to 180C/fan 160C/gas 4. Line a 23cm x 30cm swiss roll tin with baking paper. In a large, clean bowl, whisk the egg whites with a pinch of salt using electric beaters until stiff. Gradually add the sugar, 1 tbsp at a time, whisking constantly, until stiff and glossy. Fold through the cornflour, vinegar and lemon zest. Spread the mixture evenly onto the baking tray and bake for 15-20 minutes or until crisp on the outside. Remove from the oven, lay down a piece of baking paper and flip the meringue over onto it, carefully pull away the used baking paper and discard, and leave to cool.