Slice a vanilla bean down the middle and use a spoon to scrape out the seeds.
Transfer the milk and vanilla seeds to a medium pot and place over medium-high heat.
Bring to a boil whisking occasionally then immediately turn off the heat and set aside to infuse for 15 minutes.
In a bowl, whisk the sugar and corn starch together then add the egg yolks and whisk until light and fluffy.
Whisk in 1/4 cup of the hot milk mixture until incorporated. Whisk in the remaining hot milk mixture, reserving the saucepan.
Pour the mixture through a strainer back into the saucepan. Cook over medium-high heat, whisking constantly, until thickened and slowly boiling.
Remove from the heat and stir in the butter. Let cool slightly.
Cover with plastic wrap, lightly pressing the plastic against the surface to prevent a skin from forming.
Chill at least 2 hours or until cooled.