In a small bowl, whisk the sugar and corn starch together.
Add egg yolks.
Whisk until smooth. Set aside.
Pour milk in a small sauce pan.
Split and scrape vanilla bean into milk.
Add the pod you just scraped.
Heat over medium low heat just until it starts to bubble around the edges.
Whisk hot milk mixture into egg mixture a little at a time.
Return to saucepan.
Cook over medium low heat, whisking continually, until it reaches a pudding consistency, about 10-20 minutes.
Remove from heat.
Whisk in butter.
Pour into a clean bowl.
Cover with plastic wrap, making sure it is touching the surface of the pastry cream.
Refrigerate until ready to use.