Line a 9"x13" with foil or parchment paper and spray with non stick cooking spray. Set aside.
In a double boiler, combine 3 cups granulated sugar, 6 tablespoons butter, and 5 ounces evaporated milk. Cook over medium heat, stirring constantly, until the mixture reaches a rolling boil.
Continue boiling for approximately 5 minutes, stirring constantly, until the mixture reaches 234°F on a candy thermometer. Remove the mixture from the heat.
Immediately stir in 1 package (12 ounces) semi-sweet chocolate chips and 1 jar (7 ounces) marshmallow creme. Stir until the chocolate chips are completely melted and the mixture is smooth.
Stir in 1 teaspoon vanilla extract. If using nuts, stir in 1 cup chopped nuts until evenly distributed.
Pour the mixture into the prepared baking dish and spread it into an even layer.
Allow the fudge to cool completely at room temperature until set.
Lift the fudge out of the pan using the foil or parchment paper. Cut into 24 squares.