In a large bowl, add chocolate chips, marshmallows, vanilla and butter. Set aside.
In a medium pan, stir sugar and evaporated milk together over low heat until boiling, stirring constantly.
Once boiling, cook for approximately 8 or 9 minutes, stirring constantly.
Next, pour mixture over chocolate chips, marshmallows, butter and vanilla. Stir until well incorporated.
Add walnuts, stir.
Pour into a 9 x 13 baking dish, lined with parchment paper or foil.
Let cool overnight. Make sure to cover with foil after it cools a bit.
Once fudge has cooled, cut into 1" squares.
To store, place into a sealed container with wax paper in between each layer.