In the bowl of a stand mixer, combine the egg whites and two tablespoons of the sugar. Whip on medium-high speed until soft peaks form.
In a small saucepan over medium-high heat, combine the rest of the sugar, corn syrup, and water. Bring the mixture to a soft boil, stirring occasionally. Let the mixture cook until it reaches 240 degrees on a thermometer.
While mixing the egg white mixture on low speed, slowly pour the syrup into the bowl of the stand mixer. Then turn it up to medium-high speed, add the vanilla extract, and whip the mixture until fluffy, about 5 minutes.
Transfer the marshmallow to a piping bag with a round tip and pipe marshmallow over the Nutella layer. Turn on your oven's broiler and place the tart in the oven. Stand at your oven door and wait until the marshmallow turns golden brown. Don't leave your oven since marshmallow can burn quickly!
Cover and chill in the refrigerator overnight, then serve!