Pastry cream: In a medium saucepan, whisk together sugar,
cornstarch, and salt. Whisk in yogurt, egg yolks, and
butter. Cook over medium, stirring with a rubber spatula,
until mixture comes to a boil, about 7 minutes. Let boil,
stirring, 1 minute. Remove from heat; transfer to a bowl.
Cover with plastic wrap, pressing it directly onto surface
to prevent skin from forming. Refrigerate at least 2 hours
and up to 1 day. Just before using, whisk until smooth.