Preheat oven to 200 degrees F. Line two cookie sheets with parchment paper.
In a stand mixer, on medium speed, beat the egg whites with the whip attachment until foamy. Add the cream of tartar and beat until the whites are at soft peak. Slowly add the sugar in a constant stream as the eggs are whipping. Add the vanilla, vinegar and cornstarch.
Beat the eggs until they are glossy and hold a peak when lifted out of the bowl. Spoon the meringue onto the prepared cookie sheets into 12 equal mounds. Use the back of the spoon to make a well in the mound of meringue. You may need to wet the spoon slightly to do this easily.
Bake the meringues for 1 hour at 200 degrees F. Turn the oven off and allow the meringues to stay in the cooling oven for another hour. This will further dry the meringue and prevent them from collapsing.
While the meringues are baking, make your lemon curd:
In a large bowl, whisk together the eggs and yolks. Whisk in the sugar. Mix in the lemon juice and zest.
Over a double boiler, on medium heat, stir the lemon mixture with a rubber spatula until it starts to thicken. This will take 5 to 10 minutes. When you lift the spatula out of the lemon curd, draw your finger across the spatula and it should be thick enough to hold the line.
Strain the lemon curd into a large dish and cover the surface directly with plastic wrap, to prevent a skin from forming. Refrigerate until ready to use, at least 30 minutes.
Once the meringues are done baking and cooled, paint the well of the meringue with the melted chocolate. Allow the chocolate to set for 15 to 20 minutes. (This can be done the day before serving.)
To serve: add a spoon of the chilled lemon curd, top with fresh berries and dust with powdered sugar.