Line 8" square pan with foil and grease foil with butter.
Mix sugar, cocoa and salt in heavy pan.
Add milk bring to a boil over medium heat, stirring constantly.
Cook until temperature reaches 230-235 or soft ball stage.
Remove from heat.
Add butter and vanilla, but do not stir for 10 minutes.
Stir with wooden spoon until it begins to lose its sheen (about 3 minutes).
Stir in nuts.
Spread into prepared pan and let cool.
Lift out foil and remove from fudge.
Cut into squares.