1. In a medium saucepan, heat milk over medium-high heat until bubbles just start to form around the edges but milk is not yet boiling. Meanwhile, in a small bowl, stir together sugar, flour, and salt. In a medium bowl, whisk together egg yolks until well combined; slowly whisk in flour mixture until thick and pasty.

2. Remove milk from heat and slowly add to egg mixture, whisking constantly. Transfer egg mixture to saucepan and place over medium heat, whisking constantly until mixture thickens and comes to a boil, about 3 minutes. Boil, whisking, for 10 seconds, and immediately remove from heat.

3. Pour mixture through a fine mesh sieve set over a small heatproof bowl; stir in vanilla. Cover with plastic wrap, pressing down on the surface to prevent a skin from forming. Refrigerate until chilled, at least 4 hours and up to 3 days.