Boil the milk. Combine the milk, vanilla extract, and half the sugar in a saucepan. Heat over medium until steaming and just about to simmer — don't let it boil.
2 cups milk, 2 tsp vanilla extract, 7 tbsp sugar
Prep the yolks. While the milk heats, whisk the egg yolks, cornstarch, remaining sugar, and 2 tablespoons of the warm milk together in a bowl until smooth.
4 egg yolks, 5 tbsp cornstarch
Temper the eggs. Gradually ladle the hot milk into the yolk mixture a few tablespoons at a time, whisking constantly. Keep going until all the milk is incorporated.
Thicken the cream. Pour the mixture back into the saucepan and cook over medium heat, whisking constantly, until it thickens and starts to bubble. Once bubbling, keep whisking on low heat for another 30–60 seconds.
Whisk off the heat. Remove from heat and whisk for another 30–60 seconds to slightly loosen the texture.
Add the butter. Stir in the butter until fully melted and incorporated.
2 tbsp unsalted butter
Taste and adjust. Add more sugar or vanilla if needed.
Chill. Press plastic wrap directly onto the surface of the cream to prevent a skin from forming. Cool at room temperature for 30 minutes, then refrigerate until needed.
Final whisk. When ready to use, break up the chilled cream with a fork, then whisk until smooth.