In a sall saucepan, combine milk, orange zest, and 2 tablespoons sugar. Scrape in vanilla seeds, and add the pod. Bring the mixture to a boil over medium-high heat, stirring occasionally. Remove from heat and coner. Set aside 10 minutes; discard orange zest and vanilla pod.
Prepare an ice-water bath in a large bowl. In a small bowl, mix together remaining 2 tablespoons sugar with cornstarch. In a medium bowl, whisk together egg and egg yolks; add the sugar mixture and continue whisking until pale yellow.
Return milk mixture to heat. Bring to a boil, scraping vanilla seeds from side of pan.
Slowly ladle half of boiling milk mixture into egg mixture, ewhisking constantly. Transfer this new mixture back to saucepan.
Set saucepan over medium heat, whisking constantly and scraping sides and edges of pan. Once mixture comes to a full boil, whisk vigorously until very thick, about 1 1/2 minutes.
Remove pan from heat. Using a rubber spatula, scrape pastry cream into a medium bowl; set over ice-water bath. Whisk butter, piece by piece, into pastry cream while still warm. Let cool completey. Lay plastic wrap on surface of pastry cream to prevent a skin from forming; wrap tightly. Store in refrigerator up to 1 day.