Put 1 1/2 cups of the milk, the sugar, and the cocoa in a nonreactive saucepan.
Bring to a simmer, over medium-high heat.
Remove from the heat.
Whisk the remaining 1/2 cup milk with cornstarch, salt, egg yolks, and vanilla in a bowl. Gradually whisk the hot milk into the egg mixture.
Return to the saucepan and cook over medium-high heat whisking constantly, until the pudding comes to a full boil.
Reduce the heat to maintain a simmer, and continue whisking until thick. (2 or 3 minutes)
Pour the pudding into 6 small cups.
Cover with plastic wrap and refrigerate for at least 4 hours or ideally overnight until set.
Top with a bit of whip cream before serving.
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