Combine butter, milk, sugar, and salt in a medium saucepan and place it over medium heat. Bring the mixture to a boil and cook for 4 minutes while stirring constantly.
Remove from heat.
Add the marshmallows, chocolate, vanilla, and nuts and stir to combine.
Beat for about 1 minute or until the marshmallows have melted and the mixture is thoroughly combined.
Pour into the prepared pan and let the mixture cool.