Prepare Ingredients and Equipment: Assemble all required ingredients and utensils. Lightly grease a 9x13-inch (23x33 cm) baking dish with butter or nonstick cooking spray, ensuring even coverage to prevent sticking.
Combine Base Ingredients: In a large, heavy-bottomed saucepan, combine the granulated sugar, unsalted butter (or margarine), and evaporated milk. Place the saucepan over medium heat and stir continuously until the sugar dissolves completely and the mixture reaches a full rolling boil.
Boil Mixture: Once boiling, maintain a steady boil for exactly 5 minutes, stirring constantly with a heat-resistant spatula or wooden spoon to prevent scorching. Remove the saucepan from heat immediately after the timer concludes.
Incorporate Chocolate: Add the semisweet chocolate chips to the hot mixture. Stir vigorously until the chocolate is fully melted and the mixture achieves a smooth, uniform consistency.
Blend Remaining Ingredients: Gently fold in the marshmallow creme until fully integrated. Add the chopped walnuts and pure vanilla extract, stirring methodically to ensure even distribution throughout the fudge base.
Transfer and Smooth: Pour the fudge mixture into the prepared baking dish. Using an offset spatula or butter knife, spread the mixture into an even layer, smoothing the surface for a polished finish.
Cool and Set: Allow the fudge to cool undisturbed at room temperature for a minimum of 1 hour, or until completely firm. For optimal results, avoid refrigeration, as gradual cooling prevents crystallization.
Slice and Serve: Once set, use a sharp knife to cut the fudge into 1-inch (2.5 cm) squares. Wipe the blade clean between cuts for neat edges. Serve immediately or store as directed.