Heat the milk

• Pour the milk into a saucepan.
• Bring to a gentle simmer over medium heat.

Whisk the yolks and sugar

• Whisk the egg yolks with the sugar until pale and slightly thickened.
• Sift in the cornstarch and whisk until smooth.

Temper the eggs

• Pour a little of the hot milk into the yolk mixture while whisking.
• Whisk in the remaining milk gradually.

Cook the cream

• Return everything to the saucepan over medium heat.
• Whisk constantly until the cream thickens and bubbles gently.

Finish and cool

• Off the heat, whisk in the vanilla extract and the butter until glossy.
• Pour into a bowl, cover with plastic wrap touching the surface and let cool completely.