Line a 9x9 baking dish with parchment paper, leaving over-hanging edges with the parchment. Set aside.
In a large saucepan over medium heat, melt butter, sugar, brown sugar and evaporated milk, stirring constantly and bringing to a full rolling boil. Continue to boil and stir constantly for 4 minutes.
Remove saucepan from heat and stir in marshmallow cream, Nutella and vanilla until smooth. To make sure the mixture is extremely smooth, you may need to use a whisk for this step.
Pour into prepared pan and, using a spatula, make sure the mixture is distributed evenly in the pan. Sprinkle with the sea salt or fleur de sel.
Refrigerate until fudge is completely set up and cooled, about 3 hours.
Once fudge is set, remove from fridge; carefully lift out the fudge by grabbing the overhanging parchment and lifting carefully.
Set it on a cutting board and cut the fudge into 1 inch squares.
Store in an airtight container.Recipe from Chef in Training