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Whisk together egg yolks and sugar in a small bowl
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In a medium pot, heat milk, cornstarch, and cocoa powder until warm and steamy - do not boil - remove from heat
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Slowly whisk 1/2 cup of the hot milk into the egg mixture and repeat until milk is reduced by half
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Add the egg/sugar/milk mixture back into the pot with the remaining milk and bring to a light boil, stirring constantly.
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Just when the bubbles begin, remove from heat, stir another 1-2 minutes
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Stir in vanilla and butter until butter is melted and let cool for a few minutes
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Place a fine mesh strainer over a bowl and pass the pudding through the strainer into a bowl - this will catch any egg pieces that may have scrambled
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Cover with plastic wrap so that the plastic touches the surface of the pudding to prevent a skin from forming
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Chill in refrigerator overnight