Transfer the Nutella from the container to a large mixing bowl using a spatula. Add about 1 cup of the milk to the Nutella and stir to combine. Once smooth, add more milk and continue stirring. Add the rest of the milk and give it a last stir. The nutella should be mostly dissolved into the milk, but don’t worry too much if you have a few wayward lumps.
In a large sauce pan, whisk the sugar and cornstarch until it’s light and powdery. Slowly add the Nutella and milk mixture. Start with a 1/2 cup and whisk until smooth. Slowly add the rest of the Nutella and milk, whisking continuously. Add the egg yolks and whisk until combined.
Turn the burner to medium heat. Bring the pot to a simmer, stirring frequently. Once the pudding is thickened and the bubbles can barely break the surface remove the pot from the heat. It should take less than 10 minutes for the pudding to thicken.
At this point, you can transfer the pudding to a bowl, add the butter and stir until it’s melted. Cover the pudding with plastic wrap and chill. Or, divide it into serving dishes. Use a 1/3 measuring cup to portion the servings. Be sure to cover with the plastic wrap touching the surface of the pudding to prevent a skin from forming.
Chill the pudding for at least an hour or up to 2 days.
To serve: garnish each serving with a dollop of whipped cream and a sprinkling of grated dark chocolate.