Whisk together 1/4 cup sugar, the yolks, and salt in a medium bowl. Whisk in cornstarch 1 tablespoon at a time.
Bring milk and remaining 1/4 cup sugar to a simmer in a medium saucepan over medium heat. Whisk a little milk mixture into egg-yolk mixture; whisking constantly, gradually add remaining milk mixture. Return mixture to pan; bring to a boil, whisking constantly.
Cook, whisking, until very thick, 2 to 3 minutes. Pour through a fine sieve into a bowl; discard solids. Press plastic wrap onto surface; refrigerate until completely chilled, at least 1 hour and up to 3 days.