1) Put the egg yolk in a medium heatproof bowl and lightly beat it.

2) In another bowl, combine the cocoa, cornstarch, sugar, and salt. Pour in ½ cup of the milk and whisk until a smooth paste forms.

3) Bring the remaining milk to a boil, then immediately remove from the heat (boiling milk scorches quickly) and whisk the cocoa paste into the hot milk. Put it back on the burner, bring it to a boil, and cook for exactly 2 minutes, whisking constantly. Remove from the heat.

4) Gradually pour about 1 cup of the hot mixture into the egg yolk, whisk it smooth, and pour it back into the pot. Bring the pudding back to a simmer and cook over medium-low heat for 2 minutes. Do not boil. Stir in the vanilla. Pour into serving dishes and chill. Cover the bowls of pudding with plastic wrap to prevent a skin from forming.