Combine milk and eggs yolks in a large bowl, stirring with a whisk until frothy. Scrape seeds from vanilla bean. Add vanilla bean seeds and pod to milk mixture.
Combine sugar, cornstarch, and salt in a medium saucepan, stirring with a whisk. Slowly add milk mixture, stirring until blended. Cook over medium heat 12 minutes or until mixture comes to a boil and begins to thicken, stirring constantly. Remove from heat; stir in butter. Remove and discard vanilla bean pod.
Pour 1/2 cup mixture into each of 10 (6-ounce) ramekins or small bowls. Cover surface of pudding with plastic wrap coated with cooking spray. Chill at least 2 hours.
Gluten-Free Baking; The Food Lover's Gluten-Free Cookbook