In a medium saucepan, add the milk and cream and heat over low heat until steaming (avoid boiling you just want this very hot). I heat my milk and cream to about 120 F.
2 (480g) cups whole milk, ½ (120g) cup heavy cream
In a heat-safe bowl, separate your eggs and add the egg yolks. Add the sugar, cornstarch, and salt to the egg yolks, and whisk until pale.
½ (100g) cup granulated white sugar, 4 large egg yolks, 3 (24g) tbsp cornstarch, ¼ tsp salt
Slowly stream the hot milk mixture into the egg mixture while whisking constantly. This tempers the egg yolks so you don't end up with cooked egg yolk lumps! Once you've whisked the hot ingredients into the egg mixture, pour the combined mixture back into your pot.
Return the pot back to the stove over medium heat, whisking constantly. This mixture will start to thicken, and then you'll see large bubbles. Continue to whisk all of the edges to avoid any burning, and cook for about 4-5 minutes until the pastry cream becomes very thick.
Remove the pastry cream from heat at this point, and add in the butter and vanilla bean paste or extract. Whisk until smooth. Allow the pastry cream to cool a few minutes, then transfer the pastry cream to a heat-safe bowl.
2 (28g) tbsp unsalted butter, 2 tsp vanilla extract
Cover the top of the pastry cream with plastic wrap pressed directly onto the surface to avoid a skin forming. Transfer the bowl to the refrigerator, and chill completely until fully set and cooled.