Line a 7 ½ x 11 inch (19x29 cm) or 9x9 inch (23x23 cm) pan with buttered aluminum foil. Cut large marshmallows into eighths (if using). Chop chocolate (if needed). Chop nuts (if using). Set prepared pan, heatproof spatula, marshmallows, chocolate chips, nuts and vanilla next to cooktop.

Combine butter, evaporated milk, sugar and salt in a medium saucepan and bring to a boil over medium heat, stirring constantly with a heatproof spoon. Continue to boil and stir for 4 minutes.

Remove pan from heat and stir in marshmallows, chocolate chips or chunks, nuts and vanilla. Stir vigorously until chocolate and marshmallows have melted and the mixture is smooth. Quickly pour into prepared pan with the help of the heatproof spatula and spread evenly before the mixture begins to harden.

Chill fudge until firm (30 minutes in the freezer or 2 hours in the refrigerator). Using the foil, lift fudge from the pan and cut into squares (¾ inch or 2 cm). Store tightly sealed at room temperature for up to two weeks.