In a bain-marie over medium heat, heat the whipping cream and milk to 55-60C.
Remove from the heat and add the Nutella stirring until melted.
In a bowl, add 1 whole egg and one egg yolk and beat with a fork to combine.
Stirring while pouring, add a bit of the Nutella mixture to the egg mixture and then pour the whole thing into the main Nutella mixture. Place the bain marie back on the stove over low heat, stirring continuously until it reaches 86C and is thick and spreadable. It should coat the back of a wooden spoon
Variation: to make chocolate ganache substitute the Nutella with 135 grams of dark chocolate, milk chocolate or a mixture of the two.
Chocolate ganache can also be flavoured with orange, tea, spices (such as cardamon, cinnamon or vanilla) or herbs by infusing the cream with the flavouring and then straining before then adding the chocolate.