1¼ cups (300 grams) milk
½ cup sugar
2 tablespoons cornstarch
½ teaspoon kosher salt
3 egg yolks
1 teaspoon vanilla extract
In a medium saucepan, scald the milk over medium-high heat (bubbles start to form around the edge of the pan, but the milk is not boiling). While the milk is heating, in a small bowl, stir together the sugar, cornstarch and salt. In a medium bowl, whisk the egg yolks until blended, then slowly whisk in the sugar mixture. The mixture will be thick and pasty.
Remove the milk from the heat and slowly add it to the egg-flour mixture, a little at a time, whisking constantly. When all of the milk has been incorporated, return the contents of the bowl to the saucepan and place over medium heat. Whisk continuously and vigorously for about 3 minutes, or until the mixture thickens and comes to a boil. At first, the mixture will be very frothy and liquid; as it cooks longer, it will slowly start to thicken until the bubbles disappear and it becomes more viscous. Once it thickens, remove the pan from the heat.
Pour the mixture through a fine-mesh sieve into a small heat-proof bowl. Stir in the vanilla, then cover with plastic wrap, placing it directly on the surface of the cream. This will prevent a skin from forming. Refrigerate for at least 4 hours, or until cold, or up to 3 days.