Cut the vanilla bean in half (we only use ½). Make a slit lengthwise on the vanilla bean and scrape off the vanilla bean with a knife.
In a medium saucepan (preferably a heavy saucepan for equal heat distribution), combine the milk and vanilla bean and bring it to ALMOST boil (small bubbles around the edges). Remove from the heat and set aside.
In a medium mixing bowl, add egg yolks and beat gently. Add granulated sugar and whisk until the mixture turns pale yellow.
Add the cake flour and whisk until combine.
Pour small amount of the hot milk mixture to the egg mixture and quickly whisk to combine. Slowly add the rest of milk while whisking.
Set a fine-mesh strainer over the saucepan and pour everything back into the saucepan. Cook the mixture over medium heat while CONSTANTLY mixing with a silicone spatula. It starts as a thin and frothy mixture and it will start to thicken. It’ll take 20-25 minutes for the custard cream to thicken.
Stir constantly, making sure to scrape off the custard on the entire bottom and the corners of the pan. Ensure that the mixture heats evenly. Lift the spatula up occasionally to check if any large bubbles are bursting at the top of the mixture. Keep cooking and stirring until large bubbles appear. Then boil and stir for 1 minute. The custard should be shiny and drops intermittently when you lift up the silicone spatula. Add the butter to combine, if you are using it. Remove the saucepan from the heat.
Transfer the custard to a tray (or flat container) and cover with plastic wrap to prevent a film from forming on top of the custard. Put the tray over ice cubes and put the ice pack on top of the custard to cool immediately, about 30 minutes. The key is to cool quickly so the custard won’t get spoiled.
When it’s cool, put the custard back into a bowl and whisk until nice and smooth. The custard is now ready to use.
You can keep the homemade custard for up to 2-3 days in the refrigerator. To prevent a film from forming on top of the custard as it cools, put and press down a piece of plastic wrap on top of the custard. If you put in the freezer, the texture of the custard will change.