Step 1 In a heavy medium saucepan, bring the milk and vanilla bean to a simmer. Meanwhile, in a bowl, beat the yolks, sugar and cornstarch to blend.
Step 2 Gradually whisk the hot milk into the egg mixture. Return the mixture to the saucepan and whisk over medium heat until it thickens and boils gently for 1 minute. Strain the pastry cream into a small bowl set over a bowl of ice water, gently stirring until the pastry cream cools. Cover and refrigerate until chilled, about 1 hour.
Step 3Stir the pastry cream to loosen, and fold in the yogurt. The pastry cream can be prepared up to 3 days in advance; cover and refrigerate.