Preheat the oven to 120 degrees Celsius.
Place the egg whites into the bowl of a super clean kitchen mixer with the whisk attachment. You can also use good ol' fashioned beaters and a bowl, but since acquiring/eternally borrowing my Mum's stand mixer, I have not looked back.
Whisk the eggs on a high speed until they are frothy and voluminous. This should take around 3 minutes. Gradually add the sugar (I add 1/4 cup at a time) and allow it to incorporate fully, before gradually adding the next 1/4 cup. Allow the sugar to whisk in for an additional few minutes, before adding the cream of tartar, following by the baking powder and cornflour. These all ensure the meringue is super dry inside. Turn the mixer down to add these - otherwise you'll be enveloped in a cloud of white air.
Finally, add the vanilla bean paste. I turn my mixer off to add this, because otherwise it tends to get entangled in the whisk attachment and not even touch the meringue. Mix to incorporate. The meringue should now be glossy and super firm - mine normally forms a solid meringue trail when I lift the beater up. Keep whisking on high if your meringue is not firm.
Over a gentle heat or in the microwave, heat the Nutella until it is pliable.
Take your piping bag and ensure the hole is about 1-2cm in width. I didn't use a nozzle for these, but you can if you are more advanced than I am at piping. Place the piping bag in a long, tall glass, and flip the edges over the side, to give you easy access to the nozzle end of the bag. Use a spoon to spread the melted Nutella all around the inside of the piping bag, from the nozzle, right to the top of the glass. Make sure you don't just plop all the Nutella into the bottom of the piping bag, or the first couple will be pure Nutella. Not that that's an entirely bad thing.
Spray a large baking tray with spray oil, and cover that with a piece of baking paper. The oil will help the paper stay in place when you are piping.
Spoon the meringue into the piping bag, flipping the sides of the bag up over the glass as you fill it up. Once all the meringue is in and the bag is full, twist the top end, to ensure no meringue can escape.
Using firm, even, pressure, pipe the meringues, leaving a little space between each. You get a feel for the type of pressure and speed you need to use quick quickly. It's weirdly kind of addictive. Continue piping until you've used all the mixture - how many you make will depend heavily on how big you decide to pipe the meringues.
Place the meringues in the oven, as far away from the heating grill as possible (ie if the heat from your oven emanates from the top, try to put them down the bottom) and set the timer for two and a half hours. I like to make these as the last thing at night, and leave them to cool in the oven.
Once the timer has gone off, crack the oven door ajar (I use a wooden spoon) and leave to cool completely. Store in an airtight container.