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.