Preheat oven to 300 degrees. Put the rack in the bottom third and the other one in the top third. Line 2 baking sheets with parchment paper and set aside. Set up a double boiler and put Nutella in the top. Warm Nutella through and set aside, while you make the meringue. Let Nutella cool.
Fit the mixer with the whisk attachment and put egg whites in the bowl Beat on medium - high until completely foamy, Add the cream of tartar and salt and continue whipping until soft peaks form. Increase the speed to high and begin adding sugar slowly. Once all the sugar is in, the meringue should be very thick (almost stiff.) Add all the Nutella. Gently fold (with a rubber spatula) in the DNutella. Don't overmix - you want a swirled effect. Drop meringue onto sheets in large dollops. Bake for 10 minutes. After 10 minutes immediately lower the temperature to 200 degrees and rotate the pans. Bake for 1 hour. After 1 hour, check the meringues, it they are completely dried out, turn the oven off and leave them in the oven for a few hours to cool with the oven. If they still look a bit "wet", then continue baking for 20 minutes or so. Either way, turn the oven off and leave the meringues in for a few hours. When you remove meringues they should be completely dry and cool and will sound hollow when you tap the bottom.