1 (9 inch) blind baked pie crust
5 large egg yolks
3 large eggs
1 1/2 cups sugar
2 vanilla beans, split and scraped
3/4 cup custard powder
1/2 cup cornstarch
6 cups whole milk
20 medium prunes, pitted and quartered

Preheat oven to 350°F.
Place eggs yolks, eggs, sugar, and vanilla bean seeds in a large bowl; whisk to combine. Add custard powder and cornstarch; whisk for 30 seconds to incorporate. Slowly add milk, whisking to combine. Strain into a large saucepan.
Place saucepan over medium-high heat. Bring mixture to a boil, whisking gently to avoid forming air bubbles; Let boil for 1 minute, whisking gently but constantly to avoid scorching. Transfer custard to the bowl of an electric mixer fitted with the paddle attachment; mix on low to cool and slightly emulsify custard.
Meanwhile, arrange prunes in tart shell; pour custard over fruit, keeping in mind you may not be able to fit all of the custard. Transfer to oven and bake until set but with a slight jiggle and a dark brown skin has formed on top, about 1 hour and 20 minutes to 1 hour and 40 minutes. Remove from oven and let cool to room temperature.
Refrigerate tart until ready to serve, up to 2 days.