Butter (or line with non-stick foil or parchment paper) a 9x13 inch baking pan
Place the sugar, salt, butter and milk in a 4 qt, heavy saucepan over medium-high heat. Bring to a full boil. Continue to boil for 6 minutes stirring constantly.
4 1/2 cup sugar, 1/8 tsp salt, 2 tbsp butter, 13 oz evaporated milk
Remove from heat and add both chocolates and the marshmallow cream. Stir vigorously and quickly until the chocolate is melted and the mixture is uniform in color. Immediately stir in the nuts.
12 oz sweet baking chocolate, 2 cups chocolate chips, 14 oz marshmallow cream, 2 cups chopped nuts
Scrape the mixture into the prepared pan and smooth the top. Refrigerate to set before cutting into squares.
Notes
I think my mother used walnuts most of the time, I prefer pecans, but you can use whatever nut you prefer. If you don't like nuts, you can omit them.
I like to add a teaspoon of vanilla at the same time as the chocolate and marshmallow cream.
The volume of a standard can of evaporated milk is now 12oz rather than 13oz. If you don't want to open a second can of evaporated milk for a single ounce, you can add 1oz of heavy cream instead.
The recipe says simply to boil the mixture for 6 minutes. Most fudge recipes specify to bring the mixture to a specific temperature so I was curious what temperature was reached at that point. In my testing, boiling for 6 minutes after the mixture reaches a full, rolling boil consistently landed in the 233° - 237° F range so if you prefer to use a thermometer you can look for a temperature in that range.
Chocolate Peanut Butter variation:
Add 4 tbsp (64g) peanut butter along with the sugar, salt, butter and milk in the saucepan.
Add 1 oz heavy cream (or additional evaporated milk) (this makes for a total of 14oz of milk/cream - peanut butter has a tendency to be drying, this helps balance that)