Whisk sugar and cornstarch in a medium saucepan to blend.

Gradually whisk in milk, then cream.

Whisk over medium heat until mixture comes to boil, then boil until thick, whisking constantly.

Whisk in peanut butter, boil until thick again, whisking often.

Remove from heat and whisk in vanilla.

Pour into prepared dishes and chill in refrigerator for at least 2 hours.