Preheat oven to 275ºF. In a clean mixing bowl, with electric mixer on med-high, beat egg whites with cream of tartar and salt until frothy, then slowly add sugar with mixer running. Continue whipping until meringue holds very stiff peaks. Rub a little meringue between your thumb and forefinger to check that sugar has completely dissolved. If not, beat a little more. By hand, fold in vinegar and cornstarch. (You can add a little vanilla or liqueur at this point, if you like.) Drop onto a parchment-lined baking sheet in 8 equal-sized discs about 1 1/2- 2 inches high and about 4-5 inches in diameter. Leave several inches of space between meringues to allow for expansion. Smooth them out, including edges. With the back of a moistened spoon, make a little well in the centre of each (that will later hold the cream). Bake for 50-60 minutes until the outside is dry and meringues are a pale cream colour. Turn oven off, leave door ajar, and let meringues cool completely in the oven. (The outside of the meringues will feel firm but as they cool, they will crack a bit and the inside will be a little mallowy.) Meringues can be made ahead and stored in a cool, dry place, in an airtight container, for a few days.
Meanwhile, in a medium saucepan over medium heat, simmer the berries with the sugar (add 1 tbsp water if using fresh berries). Cook until it looks like jam, berries have popped and mixture has thickened. Set aside to cool. Whip the cream, adding a touch of vanilla or liqueur if you like. Assemble meringues by putting a dab of whipped cream on the dessert plate to anchor the meringue. Fill indentation with cream. Top artfully with berry compote and a sprig of mint. Serves 8.