1/2 cup heavy cream
1/2 cup whole milk
1/4 cup granulated sugar
Pinch of fine sea salt
3 large egg yolks
1 1/2 tablespoons cornstarch, mixed with 2 tablespoons water
1 teaspoon pure vanilla extract
2 tablespoons (1/4 stick) cold unsalted butter, cut into pieces
1. In a saucepan, stir together the cream, milk, 2 tablespoons of the sugar, and the salt. Warm over medium-low heat until gently simmering, stirring to dissolve the sugar.
2. In a heatproof bowl, whisk together the egg yolks and the remaining 2 tablespoons of sugar. Whisk in the cornstarch mixture. Slowly pour about half of the warm cream mixture into the yolk mixture, whisking constantly.
3. Scrape the yolk mixture into the saucepan. Cook, stirring constantly, until the custard is thickened and glossy, about 3 minutes.
4. Remove from the heat and stir in the vanilla extract and the butter until melted. Pour the pastry cream through a medium-mesh sieve into a bowl, press a piece of plastic wrap directly onto the surface of the custard, and let cool completely. (You can cool the custard down quickly by setting the bowl in a larger bowl filled with ice water.)
5. Refrigerate until chilled before using; the pastry cream can be kept in the refrigerator, covered, for up to 3 days.