In a very large bowl, beat the cream cheese until soft.
Beat in the sugar and cornstarch, turning off the mixer and scraping down the sides of the bowl as necessary.
Mix in the Nutella until smooth, followed by the sour cream.
Beat in the eggs until just combined. Do not continue mixing after the eggs have been incorporated.
Place the springform pan (still wrapped in aluminum foil) in the middle of a large roasting pan.
Pour the cheesecake batter into the pan and smooth the top.
Pour the boiling water into the roasting pan so that there's about 1/2 - 1 inch of water in the roasting pan.
Place the entire thing in the oven to bake for 55-65 minutes, or until the top looks just set, but there's still a wobble in the middle of the cheesecake.
Remove the water bath from the oven and gently trace around the outside of the cheesecake with a thin sharp knife.
Cool the cheesecake in the water bath until it's room temperature, then remove from the roasting pan. Cover, and refrigerate for at least 6 hours.