In a medium bowl, whisk together the egg yolks, ½ cup granulated sugar and cornstarch until thick and smooth; set aside
In a medium saucepan combine the milk, vanilla bean split in half and the seeds scraped out and the remaining ¼ cup granulated sugar and bring to a simmer.
Once the milk is brought to a simmer, remove the saucepan from the heat and slowly pour about half of the hot milk into the egg mixture while whisking constantly to *temper the eggs. (temper means to slowly warm the eggs by adding a small amount of milk to the egg milk to bring them to the same temperature)
Once the eggs are tempered, pour the egg mixture back into the saucepan with the remainder of the milk, continue whisking the egg and milk mixture to avoid the eggs from curdling.
On low to medium heat, whisk the custard vigorously until it starts to thicken and begins to bubble. While whisking, let the custard come to a slow boil, lower the heat and allow the custard to cook for another 2-3 minutes.
Remove the saucepan from the heat, pour it into a glass bowl and immediately press plastic wrap to cover the surface of the custard to avoid a skin from forming.
Allow the custard to cool completely in the refrigerator for 1 hour
Once the custard is chilled, whip one cup of heavy cream to stiff peak and fold it into the pastry cream to make a smooth and creamy filling.