Begin by whipping the egg whites with a dash of salt until they reach a stiff consistency. Gradually incorporate the sugar into the mixture while continuing to beat the egg whites until they become stiff and glossy.
Next, add the yolks individually, being sure to incorporate each one meticulously and at a slow pace. Mix in the corn starch, vanilla extract, and finely grated lemon zest with a plastic spatula.
Meanwhile, thoroughly heat up the milk, either in a jug for easier pouring or by transferring it to a jug after heating. Slowly pour the heated milk over the egg mixture while using an electric mixer in your right hand and holding the jug in your left hand if you are right-handed. Pour the milk in a thin stream while continuously mixing.
Transfer the egg-milk mixture back into the jug or pan in which you heated the milk, using a large pot if necessary. Whisk the mixture continuously on low heat until it reaches the consistency of a pudding. This will take around 3-4 minutes depending on the size of the pot. Make sure to regularly scrape the bottom and sides of the pot with the whisk to prevent the custard from sticking.
At this point, there will likely be a lot of foam on top of the custard, making it difficult to see. However, you will feel the custard thicken as you stir it. The foam will disappear once you begin to stir in the butter.
Remove the pot from the heat and gradually add the cubed butter, one or two pieces at a time, while whisking continuously to help it melt and mix into the custard.