Preheat the oven to 300 degrees. Line 2 baking sheets with parchment paper. You can trace 4 4-inch circles (a ramekin works well) onto each piece of parchment to get more perfect circles if desired. Place the traced side down on the baking sheet.
Separate the egg whites from the yolks, placing the whites in the bowl of an electric mixer fitted with a whisk attachment and reserve the yolks for later. Add the salt to the egg whites and beat, starting on low, and then increasing the speed to high, until soft peaks form. Add the 1 cup superfine sugar to the eggs with the mixer going, one tablespoon at a time. Whip until stiff, glossy peaks form. Add the cornstarch, lemon juice and vanilla and beat in.
Place 8 equal dollops of the egg white mixture onto the prepared baking sheets. With the back of a spoon, spread the egg whites into circles and create a well in the center of each. Make sure the edges of the meringues are higher than the center. Bake for 1 hour or until crispy and dry. Let cool on a wire rack.
While the meringues are baking, prepare the orange curd.
Place an inch of water in a pan that is wide enough to hold a mixing bowl on it without the bottom of the bowl touching the water and bring to a simmer.
In the mixing bowl, beat the egg yolks and 2/3 cup sugar for 2 minutes on medium speed. Beat in the orange juice and zest. It will look curdled; this is okay. Place the mixing bowl over the simmering water and stir constantly with a spatula, making sure to scrape the sides and bottom of the bowl as you stir. Cook until thickened enough to coat the back of a spoon, about 6 or 7 minutes. Whisk in the pieces of butter one at a time, making sure to whisk until each piece of butter is incorporated before adding the next. Pour the orange curd into a bowl, let cool and refrigerate until cold.
Just before serving, whip the cream until stiff peaks form. Whisk half of the cream into the orange curd until smooth. Gently fold in the remaining whipped cream, leaving the mixture streaky. Spoon the filling into the meringues and serve.
Optional garnishes: Orange slices Berries Mint leaves