Line 8-or 9-inch square pan with foil, extending foil over edges of pan. Butter foil.In heavy 4-quart saucepan, stir together sugar, cocoa and salt; stir in milk. Cook over medium heat, stirring constantly, until mixture comes to full rolling boil. Boil, without stirring, until mixture reaches 234°F on candy thermometer or until small amount of mixture dropped into very cold water, forms a soft ball which flattens when removed from water. (Bulb of candy thermometer should not rest on bottom of saucepan.)
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