Heat the oven to 250 degrees F (225 degrees F if you have a convection oven). Line two sheet pans with parchment paper or silicone baking mats.
In a stand mixer fitted with the whip attachment, beat the egg whites on low to medium-low speed until they form soft peaks, 2 to 3 minutes. Increase the speed to medium and beat in the sugar 1 teaspoon at a time. Beat until the mixture is glossy and thick; it will resemble marshmallow cream.
Remove the bowl from the mixer and scrape any meringue off the whisk. Sift the cornstarch over the meringue, add the vinegar and vanilla, and gently fold until everything is incorporated.
Dip a very large spoon into warm water. Let excess water drip off, then scoop into the meringue. Drop the meringue onto one of the prepared pans; the shape of the spoon is oval, so when you place the meringue on the sheet pan, it should resemble a jumbo egg. Continue scooping, rinsing the spoon in warm water after every scoop for neat, smooth meringues.
Bake the meringues into the oven and bake until they are very light and dry and easily lift off the pan, at least 90 minutes. If they start to color slightly, lower the oven by at least 25 degrees F. Don't rush the meringues; when in doubt, leave them in longer. Turn off the oven (don't open the door). Let the meringues cool slowly in the oven for 30 to 40 minutes. Remove the pans from the cool oven. The meringues are ready to be filled when totally cooled.
Fit a piping bag with a doughnut-filling tip or a plain number 5, 6 or 7 tip. Fill the bag with the Nutella or (or chocolate, vanilla or butterscotch pudding). Flip each egg over and, with a melon baller, make a hole just large enough for the tip of the piping bag to fit. Squeeze the filling into an egg just until you can see that it's starting to come out. Flip the egg onto a serving platter. Continue until all the eggs are filled.
Instant birthday party! Mark my words, this will become the new birthday cake.