In a medium saucepan, heat the whole milk over medium heat until it just begins to simmer, then remove from heat.
In a mixing bowl, whisk together the granulated sugar, egg yolks, cornstarch, and salt until the mixture is smooth and pale.
Gradually pour about half of the hot milk into the egg yolk mixture while whisking constantly to temper the eggs.
Pour the tempered egg mixture back into the saucepan with the remaining milk.
Return the saucepan to medium heat and cook, stirring constantly with a wooden spoon or whisk, until the custard thickens and coats the back of the spoon, about 5 to 7 minutes. Do not let it boil.
Remove the saucepan from heat and stir in the vanilla extract.
Strain the custard through a fine-mesh sieve into a clean bowl to remove any lumps.
Divide the custard evenly into 4 individual serving cups.
Cover each cup with plastic wrap, pressing the wrap directly onto the surface of the custard to prevent a skin from forming.
Refrigerate the custard cups for at least 2 hours until fully chilled and set before serving.