Preheat oven to 275F (135C). Line a baking sheet with parchment paper. Draw a 9 inch circle on paper. To make meringue, beat egg whites with salt until stiff peaks form. Gradually beat in superfine sugar, then continue to beat 5 minutes. Fold in cornstarch and vinegar. Using a metal spatula, spread meringue mixture evenly over circle on prepared baking sheet.
Bake in oven 10 minutes. Lower heat to 225F (105C) and cook 45 minutes more. Turn off heat and leave meringue in oven to cool slowly 1 hour at least. I usually leave it in all night. Remove from oven and carefully transfer to a plate.
Make sure your fruit is thawed at this point.
To make filling, whip cream with sugar until thick. You can cut up 1 kiwi into the whipped cream if you like. I normally do, but make sure your kiwis are sweet before you do, otherwise you can ruin the whipped cream.
Carefully spoon cream mixture over meringue, smoothing with a metal spatula. Decorate top with fruit. I slice the kiwis and line them all around the edge. Then I put the strawberries inside the kiwi line and in the very center I put the blueberries. Serve immediately cut in wedges.