Line a 13x9x2-inch pan with foil (or parchment paper) leaving a two-inch overlap on the long sides for lifting the fudge out of the pan.
In a medium bowl mix the chocolate chips, walnuts, and vanilla extract. Set it aside to be ready to quickly pour it into the boiled mixture.
Attach a candy thermometer to a heavy 3-quart saucepan and mix in the butter, sugar, and evaporated milk over medium heat. Bring it to a full rolling boil.
Stir CONSTANTLY while it continues to boil for 5-7 minutes more until it reaches 234 degrees on a candy thermometer.
Remove the pan from the heat and QUICKLY stir in the chocolate chips, walnuts, vanilla mixture, and then the marshmallow crème. Mix it well, being sure to mix and stir reaching the bottom of the saucepan.
QUICKLY pour the fudge into the prepared foiled-lined pan.
Cool the fudge in the pan on a wire rack at room temperature until the fudge has hardened.
Grab the two overlapping foil pieces and lift the fudge onto a cutting board. Cut the fudge into 48 small bars.
Store the fudge covered on top of the counter for about 2 weeks or in the refrigerator for 3 weeks.