In a medium bowl, beat on high speed until thick and pale yellow, about 2 minutes:
3 tablespoons sugar
5 teaspoons cornstarch
1 tablespoon all-purpose flour
1 large egg
1 large egg yolk
Meanwhile, bring to a simmer in a medium stainless-steel or enamel saucepan:
1 cup low-fat milk
Gradually pour about one-third of the hot milk into the egg mixture, whisking to combine. Scrape the egg mixture back into the pan and cook, whisking constantly and scraping the bottom and corners of the pan to prevent scorching, over low to medium heat until the custard is thickened and begins to bubble. Then continue to cook, whisking, for 45 to 60 seconds. Using a clean spatula, scrape the custard into a clean bowl and stir in:
1/2 teaspoon vanilla
Cover the surface of the custard with a piece of wax or parchment paper and refrigerate to thicken. This keeps, refrigerated, for up to 2 days.
