Use this recipe when making our Berry Tartlets.
Bring milk, vanilla bean, and zest to a simmer in a medium saucepan over medium heat. Remove from heat. Cover; let steep 20 minutes.
Put yolks and sugar in the bowl of an electric mixer fitted with the whisk attachment. Beat on high speed until pale and fluffy, about 2 minutes. Add flour; mix until incorporated.
With mixer on low speed, add 1/2 cup steeped milk. Return milk-yolk mixture to pan; whisk. Heat over medium heat, whisking constantly, until mixture is thick enough to coat the back of a spoon, about 4 minutes. Pass through a fine sieve into a bowl set in an ice-water bath. Let cool completely, stirring occasionally. Refrigerate in an airtight container until ready to use, up to 2 days.