1 1/3 C. Milk
2/3 C. Heavy cream
1 1/2 T. Vanilla paste
6 egg yolks
2/3 C. granulated sugar
1/4 C. Cornstarch
1 T. Butter
Bring milk, cream, and vanilla bean paste to a gentle simmer in low heat in a medium sauce pot (3 qt or similar). Whisk together egg yolks and sugar, then whisk in cornstarch. Do these separately to make it easier to combine. When milk is steamed, temper eggs mixture with the steamed milk. Return all of this to the pot and increase heat to medium. Whisk nonstop until mixture just starts to thicken.
Remove from heat, add butter (and additional vanilla or any extract/emulsion you desire) and keep whisking until butter is incorporated fully and mixture is very smooth. Immediately remove the custard from the pot and put in a glass bowl, topping the custard with cling wrap pressed firmly against the top of the custard (so a film doesn't appear on top). Refrigerate immediately.