In a medium stainless steel bowl, whisk egg yolks and sugar until well blended, about 2 minutes.
In a medium sauce pan, bring milk and vanilla bean to a boil over medium heat. Remove pan from heat. If using a vanilla bean, let steep with milk for 5 minutes, then scrape out the beans using a sharp knife and add to the milk (discard pod). Slowly whisk hot milk into yolk mixture.
Return custard to sauce pan and stir over medium heat until custard thickens. The proper consistency can be determined using a "spoon test." Coat a spoon with the sauce and run a finger across the back of the spoon. If the sauce holds a line on the back of spoon, it is ready to be removed from the heat. It takes about 4 - 6 minutes. Do not boil the sauce. Strain sauce and transfer to a bowl. Allow sauce to cool, then cover and refrigerate.