Combine 200 ml of milk and the vanilla extract in a saucepan and set on the stove on medium heat.
Aside, combine the yolks with the sugar, cornstarch and 50 ml of milk. Beat with a hand mixer for a couple of minutes. You can do this while the milk is warming up, keeping an eye out for it to make sure it doesn't boil.
When the milk is about to boil, pour the egg mixture in, lower the heat and stir (preferably with a hand whisk) until it thickens. Keep stirring until the mixture boils. When you get a bubble of air steaming up from the bottom of the pot, you can remove it from the heat. Cover the cream with cling film (directly in contact with the cream, no air in between) and let cool to room temperature.
Notes
Right after pouring the egg mixture into the warm milk, you may notice it takes some time to thicken, but once it starts, it will go pretty quickly. It is very important that you keep whisking to prevent lumps. If you get lumps, once you remove it from the heat, you can give it a quick go with a hand blender and you'll regain a nice smooth texture.
You need to make sure you bring the custard to boiling temperature, that is when the big air bubbles start puffing out on the surface. Any sauce thickened with cornstarch needs to reach boiling, otherwise you may end up with a starchy aftertaste when it cools. At the same time, you need to make sure you remove it from the heat as soon as the first air bubble has come up, as the temperature in the pot is so high now that you risk burning the cream at the bottom of the pot.
The nutritional information provided is made with an online calculator and only meant to be used as a guideline.