Remove pan from the heat and immediately add chocolate chips. Stir, stir, stir, until chips are mostly melted then add the marshmallow creme and vanilla. Stir until well combined then fold in 1 cup of the candied walnuts.
Transfer the fudge mixture to the prepared pan, smooth the surface, then sprinkle with remaining walnuts, pressing them in lightly. Set aside until completely cooled, 3-4 hours. You can also refrigerate the fudge at this point for quicker cooling.
When cool, pull the fudge out of the pan with the foil overhangs. With a long, sharp knife, cut into 1-inch strips. Cut each strip into 1-inch squares. I like to run my knife under hot water then dry with a paper towel between slices.
See Café Tips above in the post for more detailed instructions and tips.
You can also use a 9x9-inch pan for a taller fudge.