Step 1Grease a 13x9-inch baking pan. Set aside.
Step 2Combine the sugar, butter, salt and milk in a large saucepan. Bring to a boil over medium heat, stirring constantly. Boil 6 minutes, stirring occasionally.
Step 3Combine the semisweet and sweet chocolate, marshmallow cream and nuts in a mixing bowl.
Step 4Pour the sugar mixture over the chocolate and beat until the chocolate is melted. Turn into the baking pan.
Step 5Cool the fudge until firm, several hours. Cut the fudge into 1-inch squares and store in an airtight container.