In a medium bowl, whisk together the egg yolks, cornstarch, ¼ cup of sugar, and ¼ cup of milk until smooth. Set the bowl on a damp washcloth near the stove to prevent it from moving.
In a medium saucepan, combine the remaining 1 ¾ cups of milk and the remaining ¼ cup of sugar. Place the pan over medium heat and warm the mixture until it is barely simmering.
Slowly pour a small amount of the hot milk mixture into the egg mixture while whisking constantly to temper the eggs. Gradually pour the tempered egg mixture back into the saucepan, continuing to whisk constantly, it will go from thin to thick quick.
Keep the heat at medium (do not turn it up) and whisk the mixture constantly until it thickens. You’ll know it’s ready when it begins to rapidly bubble and become thick, like a thicker pudding consistency.
Remove the saucepan from the heat immediately. Transfer to a heat safe bowl. If there are any bits of scrambled egg, strain the pastry cream through a fine mesh sieve into the bowl.
While the pastry cream is still warm, whisk in the vanilla extract, a pinch of salt, and butter until fully combined. Taste for salt, add a pinch if more is needed. Place a piece of plastic wrap directly on the surface of the cream to prevent a skin from forming. Refrigerate until completely cold before using.
Once cold and thick, whisk before using.