How to Make It
Whisk together sugar, egg yolks, and vanilla in a medium bowl until mixture is pale yellow and “makes ribbons,” 3 to 4 minutes. Add flour; whisk until smooth.
Bring milk to a boil in a medium saucepan over medium, about 3 minutes. Gradually add milk to egg yolk mixture, whisking constantly. Return mixture to saucepan. Bring to a boil over medium, whisking constantly, about 3 minutes. Boil mixture, whisking constantly, 1 minute. Transfer mixture to a medium bowl; press plastic wrap directly onto surface. Let cool to room temperature, about 1 hour. Mixture can be chilled, covered, for up to 3 days.