Whisk the egg whites with the salt until they're holding firm peaks but are not stiff. Gently add in the sugar, spoonful after spoonful, still beating, until you've got a bowl full of gleaming, satiny, snowy meringue. Sprinkle the cornflour, a few drops of vanilla and the vinegar on top and fold in to combine.
Draw 6 circles of approximately 10cm circles (using a pint glass as a guide, if this helps) on each of the parchment-lined sheets.
Spoon the meringue onto the baking parchment into the delineated circles, and spread and smooth to fill. You want to make the meringue slightly higher at the rims, or just use the back of the spoon to make an indentation in the centre to hold the cream and fruit later.
Put into the oven, turn it down to 150ºC/gas mark 2, and bake for 30 minutes. Turn the oven off and leave them in for another 30 minutes, then take out of the oven to cool. I just transfer them, on their baking parchment, to wire racks.
When you want to assemble them, dollop cream into the indentation, and smooth it with the back of a spoon, leaving the odd peak. Place, one by one, a few blackberries and a few raspberries so that they look well filled but not crammed. Dust with icing sugar.