In a small saucepan add 1 cup of milk, sugar, butter and vanilla. Heat until it starts to boil.
Meanwhile, in a container, mix the cornstarch with the rest of the milk and the egg yolk. Mix until the cornstarch and egg yolk dissolve in the milk.
Pour the milk mixture with cornstarch into the saucepan once the milk that was in the saucepan is boiling and mix quickly with a wire whisk until thickened and just right.
Serve immediately and decorate with cinnamon powder.
NOTE: You can use your favorite milk and replace the sugar with your favorite sweetener, remembering that depending on the milk and sweetener you use, the flavor will vary. You can add cinnamon sticks or lemon peels when cooking to add flavor. Egg yolk is optional.