Lightly grease an 8x8-inch square baking pan with cooking spray and line it with parchment paper; set aside.
In a medium saucepan over medium-low heat, add sweetened condensed milk, Nutella, chocolate chips, and salt.
Stir constantly until all the ingredients are blended, the chocolate is melted and the mixture is smooth. Add heavy cream by the tablespoon until the consistency is reached.
Add pure vanilla extract and stir well before gradually adding chopped walnuts; stir to blend.
Quickly pour the mixture into the prepared baking dish and, using an angled spatula, spread and level it out evenly.
Allow to cool a little before transferring to the fridge until the fudge is set, about 3 to 4 hours. Cut into small squares before serving. Makes 64 1-inch squares.
Notes
Footnote: Canadian cans are 300 ml or 10 oz. while Americans say 14 oz. Actually, both have the same amount in the can which is 1 ¼ cups.