Generously butter an 8×8-inch baking dish or coat it with nonstick spray; set aside. In a medium saucepan, blend the sugar, evaporated milk, and butter over medium-low heat, stirring frequently until the mixture reaches a rolling boil. Maintain the boil for 5 minutes, stirring without pause to prevent scorching—adjust the heat as necessary to keep a steady boil, aiming for a soft-ball stage of 235°F. Take the pot off the heat, add the semi-sweet chocolate chips and marshmallow creme, and stir thoroughly until smooth and uniform. Incorporate the chopped nuts, saving a small portion to sprinkle on top if desired. Transfer the mixture into the prepared dish, smooth the surface, and allow it to cool completely before slicing to serve.