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1
Preheat oven to 250F and line 2 large baking sheets with parchment.
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2
Swirl whites in a metal bowl set over a saucepan of simmering water until barely warm to the touch.
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3
Beat whites with salt in a standing electric mixer until they hold soft peaks.
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4
Beat in 2 cups superfine sugar and continue beating until mixture holds stiff, glossy peaks (see cooks' note, below).
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5
Stir together remaining 1/2 cup superfine sugar and cornstarch.
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6
Beat into meringue, then beat in vinegar.
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7
Spoon 8 mounds of meringue (each about 2 inches high and approximately 1/2 cup) 1 inch apart on each lined baking sheet.
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8
Divide any remaining meringue among mounds.
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9
Bake in upper and lower thirds of oven, switching position of sheets halfway through baking, until crisp but still soft inside, 1 to 1 1/4 hours total.
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10
If meringues are still not crisp after 1 1/4 hours, turn off oven and cool in oven 1 hour.
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11
Transfer from parchment to racks to cool.
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12
(Meringues may stick if cooled completely on paper.)
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13
Just before serving, toss strawberries with lime juice and 1/4 cup granulated sugar and let stand, tossing occasionally until sugar is dissolved, 10 minutes.
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14
Add blackberries and toss to coat.
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15
Beat cream with vanilla and remaining 1 1/2 tablespoons granulated sugar with cleaned beaters.
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16
Tap meringues gently with back of a spoon to create indentations, then mound some whipped cream and berries onto each.
