place cornstarch and 1/2 cup of milk in a bowl and whisk till cornstarch is dissolved completely. Add 4 yolks and whisk to combine.
In a small saucepan heat remaining 2 cups milk, sugar, and vanilla. Stir to dissolve sugar. do not bring the milk to a full boil, just until bubbles form around the adges. turn off the heat.
Temper the eggs- add 1/3 cup of the hot milk to the egg mix, in a slow stream while whisking constantly. DO NOT pour it directly in or you will scramble the eggs. Add another 1/3 cup of milk in the same way. The eggs should now be hot.
Pour the hot eggs into the remaining milk in the saucepan and bring the heat up again. Add the cream. Whisk constantly for 2/3 minutes until the mixture changes to a custard. (it will be the consistency of thick Pantene conditioner).
Pour the custard into a shallow glass dish. Cover the top with plastic wrap, making sure thw wrap directly touches the top of the custard, corners and all. This will prevent a skin from forming on the top. Refrigerate until cold- a couple hours. I like to make this the day before i plan on using it.
Optional- for a creamier custard, whip for about 30 seconds before placing in a piping bag!