In a medium sized mixing bowl, add the egg yolks and cornstarch. Mix well with a fork or whisk until well incorporated and no clumps are present. Set aside.
In a large saucepan over medium heat, add the sugar, heavy cream, and half and half. Heat until just bubbling.
Once bubbling, use a measuring cup to scoop out about a half a cup of the hot cream mixture and slowly pour it into the egg yolk mixture. Do this slowly so the eggs don't scramble, and mix constantly as you're pouring the cream mixture in.
You are going to keep slowly adding some of the cream mixture to the egg yolks to temper the yolks and keep them from scrambling. Do this slowly!Keep tempering until you've poured about half of the cream mixture into the yolk mixture. Then, pour the yolk mixture back onto the stove and let it come to a bubble.
Once it comes to a bubble, it should start to thicken. Keep stirring and once it is thick, remove from the heat.
Then add in the butter, and mix well. Next add in the salt and vanilla and mix well.
Transfer the vanilla pastry cream to a large bowl and cover with plastic wrap so a skin doesn't form on the top. Place in the fridge and let cool for at least 2 hours, up to overnight.
(This pastry cream can be made a day ahead and kept in the fridge until ready to assemble.)