This makes enough to fill two nine-inch tart shells, three 3-by-14-inch rectangles, or up to four dozen 3-inch tartlets. Use this recipe when making our Berry Tartlets.
Combine whole eggs, yolks, and 1/2 cup sugar in the bowl of an electric mixer fitted with the whisk attachment. Beat on medium-high speed until mixture is pale yellow and thick, about 3 minutes. Turn off machine. Sift in cornstarch; beat on medium-low speed until combined, scraping down sides of the bowl.
Fill a large bowl with ice and water. Bring milk, remaining 1/4 cup sugar, and vanilla bean and seeds to a boil in a medium saucepan. Remove vanilla bean. Whisking constantly, slowly pour half of milk mixture into egg mixture; whisk until smooth. Pour mixture back into saucepan. Set over medium heat, and whisk until mixture reaches consistency of mayonnaise, 2 to 3 minutes.
Transfer mixture to a large bowl. Add butter 1 tablespoon at a time, stirring until melted after each addition. Set bowl over ice bath until chilled. Cover surface directly with plastic wrap to keep a skin from forming; let chill overnight or at least 1 1/2 hours before using.
This pastry cream can be made ahead and chilled for up to two days.