In a heavy 4-quart saucepan over medium heat combine sugar, cocoa, milk, and salt. Bring to a boil, stirring constantly. Continue cooking, not stirring, until mixture reaches 235° on a candy thermometer, or until it reaches soft ball stage**. Remove from heat; stir in butter and vanilla. Let stand to cool for about 1 hour, until mixture comes to room temperature. Wit a wooden spoon, beat until fudge thickens and loses its gloss. Stir in toasted walnuts and spoon into a buttered 8-inch square pan. Cool completely then cut into 1-inch squares. Makes about 64 pieces.
*To toast nuts, spread out in a single layer on a baking sheet. Toast in a 350° oven, stirring occasionally, for 10 to 15 minutes. Or, toast in an ungreased skillet over medium heat, stirring, until golden brown and aromatic.
**To Test for Soft Ball Stage
A small amount of syrup dropped into chilled water forms a ball, but is soft enough to flatten when picked up with fingers (234° to 240°).