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Add the beaten egg yolks to a heatproof bowl. Set aside.
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Place the bourbon, milk, sugar, and flour in a saucepan over medium heat. Whisk until the mixture starts to boil and has thickened.
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Slowly pour the mixture into the bowl with the egg yolks, whisking constantly to temper them.
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Add the mixture back into the saucepan and stir in the butter.
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Stir in the vanilla bourbon extract.
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Transfer the mixture back into the heatproof bowl. Let sit for 5 minutes.
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Cover the bowl with plastic cling film, making sure the plastic touches the surface of the pastry cream.
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Chill for at least 2 hours.
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Whip the heavy cream and fold into the vanilla bourbon cream.
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Chill until ready to use.