In a large saucepan, combine and stir cocoa, salt, sugar, and milk until boiling. Reduce to medium heat and cook until soft ball forms when dropped in a cup of cold water. Remove from heat and add butter and vanilla. Let cool approximately 10-15 minutes. When cooled, beat mixture until it loses some of its gloss and begins to thicken. Add nuts if desired. Pour into glass dish or casserole dish that has been coated with butter. Put in refrigerator to harden. Once set, cut into squares and serve.