In a small bowl, lightly beat egg whites and set aside. In a large bowl, combine 2/3 cup of cocoa and cornstarch. Whisk 3/4 cup of milk into cocoa mixture until completely smooth.
In a large heavy saucepan, combine remaining milk, sugar and salt. Mix well. Bring to a boil over high heat, whisking constantly. Remove pan from heat.
Gradually whisk 1 cup of hot cocoa mixture into the egg whites. Pour mixture back into pan. Cook over medium-low heat for 2 minutes, whisking constantly. Do not boil. Remove pan from heat.
Add vanilla; blend well.Pour pudding into serving dishes. Cool to room temperature. Cover and chill for 1 hour.
Garnish with berries.
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