In a 1 1/2 qt. Casserole, stir sugar, cocoa, salt, milk and vanilla together until partially blended (it will be too stiff to blend thoroughly). Put butter over top in center of dish. Microwave at high for 2 minutes, or until smooth. If all butter has not melted in cooking, it will as mixture is stirred. Blend in nuts. Pour into a lightly-buttered 8x4x3" dish. Chill 1 hour in refrigerator or 20 to 30 minutes in freezer. Cut into squares. Makes about 36 squares.