Step 1

Place egg whites in a large bowl; let stand at room temperature 30 minutes.

Step 2

Preheat oven to 225°. Add vinegar, vanilla, salt and cream of tartar to egg whites; beat on medium speed until foamy. Gradually add sugar, 1 tablespoon at a time, beating on high after each addition until sugar is dissolved. Continue beating until stiff glossy peaks form.

Step 3

Cut a small hole in the tip of a pastry bag or in a corner of a food-safe plastic bag; insert a large star tip. Transfer meringue to bag. On a parchment-lined baking sheet, pipe meringue into twelve 3-in. round disks, building up the sides with meringue to form shells. Bake 45-50 minutes or until set and dry. Turn off oven (do not open door); leave meringues in oven 1 hour. Remove from oven; cool completely on baking sheet.

Step 4

Meanwhile, in a small heavy saucepan, mix sugar and potato starch. Whisk in water and 1/2 cup lemon juice until smooth. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat to low; cook and stir 2 minutes longer.

Step 5

Remove from heat. Stir a small amount of hot mixture into eggs; return all to pan, stirring constantly. Bring to a gentle boil; cook and stir 2 minutes. Remove from heat. Gently stir in lemon zest and remaining lemon juice. Transfer to a small bowl; cool without stirring. Refrigerate, covered, until cold.

Step 6

Spoon curd into meringue shells. If desired, top with whipped topping and berries.