Bring 2 cups of the milk to a boil in a medium saucepan. While it is heating, combine sugar, cornstarch, salt and vanilla bean {if you’re replacing it with extract, don’t add it yet; you can toss the leftover vanilla bean pods in the pot with the simmering milk for an extra vanilla boost} in the bottom of a medium, heatproof bowl. Gradually whisk in the remaining 2/3 cup whole milk, a little at a time so lumps do not form, then whisk in the egg. Once milk is boiling, very gradually add it to the cornstarch mixture in the bowl, whisking the whole time.