Place a fine-mesh sieve in a small shallow baking dish or bowl.
In a medium saucepan, heat milk over medium heat until steaming. (Do not boil.)
In a medium bowl, whisk together sugar, cornstarch, lemon zest, flour, and salt. Whisk in egg yolks until well combined. (Mixture will be very thick.) Gradually whisk in half of warm milk. Pour sugar mixture into remaining warm milk in pan. Bring to a boil over medium heat, whisking constantly. Cook, whisking constantly, until mixture is thickened and no longer tastes starchy and an instant-read thermometer registers 185°F (85°C), about 3 minutes.
Remove mixture from heat. Stir in butter until well combined. Pour and press mixture through prepared sieve into baking dish or bowl; smooth into a thin layer. Cover with a piece of plastic wrap, pressing wrap directly onto surface of pastry cream to prevent a skin from forming. Refrigerate until thoroughly chilled, about 1 hour or up to overnight. Stir until softened and smooth before using.
Recipe by Bake from Scratch at https://bakefromscratch.com/strawberry-brioche-tarts/