Mix the egg yolks with the cornstarch then slowly drizzle 1/3 cup of half-and-half into the bow while stirring.
Place the remaining half-and-half, sugar, and vanilla in a medium-sized pan and place over medium heat.
Once the liquid starts to simmer, remove from heat and drizzle 4 tablespoons into the egg yolk mixture while stirring. Run the yolk mixture through a sieve over a clean bowl.
Return the pan back over the stove to medium heat, and pour the egg yolk mixture into the pan.
Using a wooden spoon or a rubber spatula, cook the cream while constantly stirring, until thickened. About 60-90 seconds.
Remove from heat, and place the pastry cream into a large bowl. Cover the surface with plastic wrap and cool to room temperature.
Place the now room-temperature pastry cream in a bowl of a stand mixer and whisk on medium speed.
Add the butter one piece at a time increasing speed to medium-high and then to high as you add the butter.
Once the butter is fully incorporated, the creme is ready to be used.