Preheat oven to 250°F. In a stand mixer, beat egg whites and salt, just until soft peaks form. Then beat in the superfine sugar. When stiff and glossy, gently fold in the sifted cornstarch and vinegar. Test if the meringue is the right consistency by inverting the bowl. If it defies gravity, you've got a winner!
On a parchment lined baking sheet, spoon the meringue, and then add dollops at a time in a ring formation. If there is leftover meringue, add a second layer of more dolloped meringue.
Using the back of a spoon, lightly smooth the surface of the ring. Bake for 2 hours. When finished, leave in the oven to cool completely.
When cooled, remove from the oven and top with whipped cream, berries, and mint.