In a medium bowl, whisk together the egg yolks, sugar, cornstarch and salt aggressively for 1-2 minutes, until the mixture becomes smooth and runny, with no more chunks.
1 large egg (OR 3 large egg yolks)*, 30 grams (3 Tbsp) cornstarch, 45 grams (1/4 cup) granulated sugar
In a medium saucepan (preferably stainless steel or nonstick – avoid aluminum), heat the milk over medium heat, just until it comes to a simmer.
240 cups (1 cup/8oz ) milk
Temper the eggs by streaming half of the milk into the egg mixture while continuing to whisk.
Dump the whole mixture back into the saucepan and cook over medium-low heat, while whisking continuously.
Within 1-3 minutes, the mixture should start to thicken, continue whisking.
Once it's starting to stick to the pan, whisk aggressively for another 30-60 seconds. Remove from the heat and pour into a bowl with the butter and vanilla.
42 grams (3 Tbsp) salted butter, 1/2 Tbsp vanilla bean paste
Allow the heat from the pastry cream to melt the butter for a second and then gently whisk it. It may separate at first but it'll come back together as you whisk it.
The pastry cream should be smooth and silky now, if you have any lumps, feel free to push it though a sieve. Allow to cool completely before moving forward. You can make this the night before and refrigerate it.