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Whisk milk, half of the sugar, and the vanilla bean seeds (and you can add the pod in there too to help infuse the milk). If you aren’t using vanilla bean seeds, and using the vanilla extract instead, you want to wait to add it at the very end.
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Bring mixture almost to a boil. Turn the heat off. Let it infuse for about 20 minutes. (If not using vanilla beans, skip this step.)
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If you let your milk mixture infuse, you want to quickly re-heat it for just a little bit before proceeding.
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Whisk egg yolks, plus half of the sugar, cornstarch, and flour in a bowl.
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Whisk until mixture is very lightened in color, and a bit runny.
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Add a couple of tablespoons of the hot milk to egg yolks, while whisking non-stop.
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Add another couple of tablespoons. Keep doing this while you whisk the mixture. You are tempering the eggs, and avoiding them to cook, by slowly raising the temperature.
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At the end you can just pour the rest of the milk in.
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Put mixture back in the pot where you heated the milk, and make sure to strain it before, so you can catch any bits of eggs that may have cooked.
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Then, use a spatula or wooden spoon to stir your custard over medium heat. Don’t stop stirring, don’t look away, you don’t want it to overcook.
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Keep cooking and stirring until it starts to get kind of grainy, and then it will start to get really thick and smooth. When the whole mixture is creamy, smooth, and thick, you can turn the heat off.
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Add butter and mix until melted and incorporated.
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Transfer mixture to a heat-proof bowl. Let it sit for about 10 minutes and then add the vanilla extract if that’s what you are using.
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Cover with plastic wrap directly on the surface of the cream. Place it in the fridge until completely cooled down.