Cut the Unsalted Butter (1/2 cup) into small pieces and mix well until it reaches a creamy consistency. Set aside.
In a separate bowl, slightly beat yolks of the Egg (2) and add Granulated Sugar (1/4 cup). Continue to whisk until the mixture is pale white and sugar is mostly dissolved.
Add Milk (3 1/2 tablespoon).
Transfer the egg yolk mixture into a small saucepan and heat over low heat, stirring frequently. Continue to cook until it’s thick like pudding. Transfer the mixture back to the bowl and bring it to room temperature.
Stir in the butter in three batches.
Add Vanilla Extract (1 teaspoon) and continue to mix until smooth.
Now you are ready to fill some eclairs!