Warm the milk: Heat milk until it’s just shy of bubbling. You don’t want a boil—scalding isn’t a badge of honor here.
Whisk the yolks and sugar: In a separate bowl, whisk yolks with sugar until pale and smooth. No lumps, please.
Make the slurry: Stir in cornstarch (or flour) so you’ve got a smooth paste that won’t clump later.
Temper the eggs: Slowly pour a ladle of warm milk into the yolk mixture while whisking. Then pour the yolk mix back into the pot. This keeps eggs from scrambling like bad reality TV.
Thicken on the stove: Cook on medium heat, whisking constantly, until the mixture coats the back of a spoon.
Finish: Remove from heat, stir in vanilla and a pinch of salt. If you’re feeling luxe, whisk in a pat of butter or a splash of cream.
Chill and enjoy: Pour into bowls, cover, and chill for at least an hour. Cold, creamy bliss.