Remove 1 tbsp. of milk and set aside then bring the milk to a boil over low-medium heat and add in the vanilla bean stalk if you are using it. If you are using vanilla extract, you can add it to the egg mixture in the next step.
While the milk is simmering, in a bowl add the egg yolks, sugar, corn starch, lemon zest (if using) and whisk well. The mixture will be clumpy, and you can now add the 1 tbsp. of milk to help loosen the mixture up.
Once the milk boils, add it gradually into the egg mixture a little bit at a time, whisking continuously.
Using a strainer, return the milk and egg mixture back into the pot and bring to boil until it thickens up. Be sure to keep whisking so the mixture doesn't clump up.
Once the custard cream thickens up, remove from heat. Add the butter and whisk.
Transfer to a glass bowl and cover with saran wrap. Allow to cool at room temperature before using.
Once the mixture cools, I like whipping the custard cream for a minute to obtain a nice creamy texture.
NOTE: If you are adding this to already baked recipes, add 1 cup of whipped (already whipped with some icing sugar) cream into the custard when whipping up to smoothen out.