Beat the egg yolks and sugar until light yellow and thick in the bowl of an electric mixer fitted with the paddle attachment, on medium-high speed. On low speed, add the cornstarch, cocoa powder, and salt. Bring the milk to a boil in a medium saucepan and, with the mixer on low, slowly pour the hot milk into the chocolate mixture. Combine well, then pour the mixture back into the pan.
Cook the mixture over low heat, stirring constantly with a whisk or wooden spoon, until thickened. If the mixture begins to curdle, remove it from the heat and beat it vigorously with a wire whisk. Remove the pan from the heat, add the chocolate, butter, vanilla, and heavy cream, and mix until the chocolate and butter are melted.
Pour into serving bowls. Place plastic wrap directly on the top of the pudding, and chill thoroughly.
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