In a medium-sized stainless steel bowl, mix the sugar and egg yolks together with a wooden spoon. (Never let the mixture sit too long or you will get pieces of egg forming.)  Sift the flour and cornstarch together and then add to the egg mixture, mixing until you get a smooth paste.  Set aside.

Meanwhile in a saucepan combine the milk and split vanilla bean on medium heat until boiling.  (The milk will foam up to the top of pan when done, so watch carefully.)  Remove from heat and add slowly to egg mixture, whisking constantly to prevent curdling.  (If you get a few pieces of egg (curdling) in the mixture, pour through a strainer.)  Remove vanilla bean, scrape out seeds, and add the seeds to the egg mixture. (The vanilla bean can be dried and placed in your sugar bowl to give the sugar a vanilla flavor.  This sugar can then be used in baking where you would like a vanilla-flavored sugar, e.g. pies, cakes, cookies.)

Place the egg mixture back into a medium saucepan and cook over medium heat until boiling, whisking constantly.  When it boils, whisk mixture constantly for another 30 - 60 seconds until it becomes very thick and it is hard to stir.

Remove from heat and immediately whisk in the liqueur (if using).  Pour into a clean bowl and immediately cover the surface with plastic wrap to prevent a crust from forming.  Cool.  If not using right away refrigerate until needed, up to 3 days.  Beat before using to get rid of any lumps that may have formed.

For a light pastry cream, just before using, fold in the softly whipped cream with a wooden spoon.