Step 1 Sift together the cornstarch and sugar.
Step 2 Combine the egg yolks and eggs in a separate bowl and set aside.
Step 3In a heavy bottomed saucepan combine the cornstarch and sugar. Stir in the milk and the vanilla bean with seeds. Heat the mixture to a simmer, stirring, and cook for 2 minutes, stirring constantly. Remove the vanilla bean.
Step 4Pour a little of the hot milk mixture into the eggs and whisk to combine. Return the egg-milk mixture to the saucepan and whisk to combine. Heat to a simmer again. Continue to cook over low heat until the mixture thickens, about 1 minute.
Step 5Remove from heat and whisk in the butter until it is fully incorporated and the mixture is smooth. Strain into a bowl, then cover the surface with plastic wrap. Cool completely in the refrigerator.