Place a piece of wax or parchment paper in the bottom and up/over the sides of a 9 x 13-inch pan.  Set aside.

In a large saucepan over medium-low heat, combine sugar, butter, 3 c. cream and salt.  Stir occasionally until butter melts.  Once butter melts, continue boiling the cream mixture until it reaches soft-ball stage (236-238 degrees F) on a candy thermometer, about 20-25 minutes.   Remove from heat and let mixture cool to about 220 degrees F.  

Place chocolate chips, marshmallow creme, 5 large marshmallows and vanilla in a large bowl.  Pour cream mixture over this.  Using a standing or hand-held mixer on low speed, blend all together until mixture is smooth.  Chocolate may become lumpy.  Add remaining 1/2 c. cream and continue to blend until smooth and warm to the touch.  Stir in walnuts.  Pour fudge into parchment lined 9 x 13-inch pan.  Smooth top, cover, and place into refrigerator for up to 2 hours to set. 

When ready to serve, lift fudge using the parchment paper.  Cut into desired portions using a sharp knife.  Store tightly wrapped in the refrigerator.  Fudge will keep for 2-3 weeks in the fridge.