In a medium saucepan, heat milk, ¼ cup (50 grams) sugar, lemon zest, and salt over medium heat, stirring occasionally, until steaming. (Do not boil.)

In a medium bowl, whisk together egg yolks, cornstarch, and remaining 4 teaspoons (16 grams) sugar. Pour hot milk mixture into egg yolk mixture, whisking constantly. Return mixture to saucepan, and cook, whisking constantly, until mixture starts to boil. (Mixture will thicken.) Cook, whisking constantly, until cornstarch flavor has cooked out, about 1 minute.

Remove from heat, and strain through a fine-mesh sieve into a medium heatproof bowl. Stir in butter until completely combined. Cover with plastic wrap, pressing wrap directly onto surface of pastry cream to prevent a skin from forming. Refrigerate until completely cooled. Whisk smooth before using.