In a large nonstick saucepan, add milk and half the sugar.
Cook on medium-high heat until mixture comes to a to a boil.
Then, reduce heat to low heat and allow mixture to simmer.
In a separate mixing bowl, whisk together remaining sugar and cornstarch.
Add whisked egg yolks to this sugar mixture and mix until smooth.
Remove 1 cup of the warm milk from the saucepan and mix that into this sugar/egg/cornstarch mixture until smooth.
Pour this mixture into the saucepan, while constantly mixing with a wooden spoon or whisk. You can bring up the heat from low to medium, if you like.
Keep stirring until custard thickens and starts sticking to your whisk or spoon.
Use a candy thermometer to ensure temperature reaches 185 degrees F. This ensures that custard does not become watery while cooling in the fridge.
Remove saucepan from heat.
Add butter, vanilla extract and mix until combined.
Transfer to a large serving bowl or separate small bowls.
Let it cool to room temperature.
Cover with plastic wrap so that it is touching the surface of the custard.
Let it cool to room temperature. Then, refrigerate overnight or for at least 6 hours. It will thicken more, as it chills.
Enjoy with a topping of fresh berries or whipped cream.