Line an 8-inch square pan with foil or parchment, leaving enough overhang to lift out the finished fudge. Lightly butter or spray the lining.

In a heavy saucepan, combine the sugar, butter, evaporated milk, pumpkin puree, and pumpkin pie spice. Cook over medium heat, stirring constantly, until the mixture reaches a full rolling boil.

Continue boiling and stirring until a candy thermometer reads 234°F, usually about 5 to 10 minutes after boiling begins. Remove the saucepan from the heat immediately when it reaches the target temperature.

Quickly stir in the white chocolate chips, marshmallow creme, and vanilla extract. Continue stirring until the mixture is smooth, then fold in the optional chopped pecans or walnuts.

Pour the hot fudge mixture into the prepared pan and smooth the surface. Let it cool at room temperature, then refrigerate for at least 2 hours or until firm.

Lift the fudge from the pan using the lining overhang and peel away the foil or parchment. Cut into small squares with a sharp knife, wiping the blade between cuts for clean edges.