How to Make It
Preheat the oven to 325° and line a large baking sheet with parchment paper. In the bowl of a standing electric mixer fitted with the whisk, beat the egg whites with the salt at medium-high speed until soft peaks form. At high speed, beat in 3/4 cup of the sugar, a few tablespoons at a time, until the whites are stiff and glossy. Beat in 1/2 tablespoon of the lemon juice along with the cornstarch and vanilla seeds. Transfer the meringue to a pastry bag fitted with a 1-inch plain tip and pipe out six 4-inch rounds, about 1 inch thick. Using the back of a spoon, make a 1 1/2-inch well in the center of each meringue. Bake the meringues in the lower third of the oven for 5 minutes. Immediately lower the oven temperature to 225° and bake for 1 hour. Turn the oven off, prop the door open and let the meringues cool completely; they should be hard on the outside and soft on the inside.
Meanwhile, in a saucepan, combine the yolks with the passion fruit puree and the remaining 1/2 cup of sugar and 2 tablespoons of lemon juice. Cook over low heat, whisking, until the sugar dissolves. Add the butter and cook, whisking, until thickened, about 8 minutes. Immediately strain the curd into a bowl. Press a sheet of plastic wrap onto the surface of the curd and refrigerate until chilled.
Set the meringues on plates and fill the centers with passion fruit curd. Serve right away, passing extra curd at the table.