Note: There are several stages the milk goes through during the cooking process. When the milk first comes to a boil there is a lot of foam. Eventually the foam subsides and after about 15 minutes, the milk turns a light beige color. As it continues to cook, the milk thickens and gets darker in color. Once the Dulce de Leche has been reduced to about 1 1/4 cups (300 ml) remove from heat and strain. Stir in the vanilla extract. Let cool before covering and storing. The Dulce de Leche can be stored in the fridge for a month.