Modified from https://marshmallowfluff.com/never-fail-fudge/
7½ oz jar Marshmallow Fluff 2½ cups sugar ¾ tsp salt ½ stick butter or margarine 5 oz evaporated milk ¾ tsp vanilla 12 oz (~2 cups) semi-sweet chocolate pieces ½ cup chopped walnuts (optional)
Grease a 9-inch square baking pan; set aside. In large saucepan combine first 5 ingredients. Stir over low heat until blended. Increase heat to medium and bring to a full-rolling boil being careful not to mistake escaping air bubbles for boiling. Boil slowly, stirring constantly, 3½ – 5 minutes to the soft ball stage*. Remove from heat, stir in semi-sweet chocolate pieces until they are melted. Stir in vanilla. Add nuts. Turn into greased pan and cool. Makes 2½ pounds.
*The soft ball stage is a test to see if the fudge has been cooked to the proper stage. Before you start cooking, fill a small dish with ice water and set aside. After you have brought the recipe to a full boil for 3½ minutes, dribble a few drops of the mixture into the ice water from a wooden spoon. After it cools in the water (about 10 seconds), you should be able to roll it into small ball with your finger tips. If you put it in your mouth, it will be slightly chewy. If it passes these tests, you are done and should remove the mixture from heat and add the remaining ingredients. Otherwise, cook for another 30 seconds and try the test again. Most of the time, cooking time will not exceed 5 minutes.
Hint: As with all fudge recipes, cooking time will vary depending on factors such as humidity, altitude, and cooktop temperature.