Place the egg yolk (use the egg white for another dish), butter, sweetener, and a few drops of vanilla flavoring in a small saucepan.

Pour half of the prepared milk into the saucepan.

Add the starch to the remaining milk and mix thoroughly until smooth (without lumps).

Place the saucepan over medium heat and heat the mixture, stirring constantly with a hand whisk or fork. It's important to achieve a smooth and even consistency. Once the contents of the saucepan begin to simmer slightly, reduce the heat and start pouring in the starch-milk mixture in a thin stream (stirring again before pouring to ensure nothing settles at the bottom). Continue stirring with a whisk or spoon at a moderate pace. Once all ingredients are combined, keep stirring continuously and increase the heat to medium. Stir the pudding until it boils and thickens, then turn off the heat. The pudding should be smooth and free of lumps. If lumps appear, try to remove them by mixing more thoroughly and pressing them with a whisk or spoon.

Transfer the finished pudding to a bowl. Enjoy it warm or cold.