Thoroughly combine cocoa, sugar, and salt in a heavy 4-quart saucepan; stir in milk.

Bring to a bubbly boil on medium heat, stirring constantly.

Boil without stirring to 234 degrees or the soft ball stage. Bulb of candy thermometer should not rest on the bottom of the saucepan.

Remove from heat; add butter or margarine and vanilla. DO NOT STIR.

Cool, at room temperature, to 110 degrees; beat until fudge thickens and loses some of its gloss.

Quickly spread in a lightly buttered 8- or 9- inch square pan; cool.

Note: For Marshmallow-Nut variation, increase cocoa to 3/4 cup and cook fudge as fudge. Add 1 cup marshmallow creme with butter or margarine and vanilla. DO NOT STIR, cool to 110 degrees, and beat 10 minutes. Stir in 1 cup broken nuts and pour into pan. (Fudge does not set until it is poured into the pan.)

Make 3 dozen squares