Line an 8x8-inch baking pan with aluminum foil and spray it all over with cooking spray.
Add the chocolate chips, sweetened condensed milk, vanilla, and Nutella to a medium saucepan and cook over low heat, stirring constantly until mixture is smooth.
12 ounces semi-sweet chocolate chips, 14 ounces sweetened condensed milk, 1 teaspoon vanilla extract, ½ cup Nutella
Remove from heat and fold in 2½ cups mini marshmallows and 1¼ cups chopped walnuts, reserving the rest to sprinkle on the top.
3 cups mini marshmallows, 1½ cups chopped walnuts
Pour the fudge mixture into the prepared baking pan and spread evenly. Sprinkle the reserved marshmallows and walnuts on top and gently press them into the fudge.
Let the fudge sit at room temperature for 6 hours or in the refrigerator for at least 4 hours until set. Cut into 25 squares.
Store fudge in an airtight container for up to two weeks.