Bring the mixture to a boil. During cooking, the foam will form; remove it from the walls bringing it back to the pot. Expert tip: if the milk isn't fresh (it usually takes longer than 24 hours to reach the supermarket shelf), add baking soda at this moment to prevent milk from curdling. Reduce the heat to medium-low and, constantly stirring with a hand whisk or a heat-resistant spatula, simmer the mixture for 1-1.5 hours.

The water content of the milk will slowly evaporate. As a result, the milk will thicken, going from a syrupy consistency to a creamy texture. Its volume will decrease by at least half, better one-thirds.  The final product has a creamy yellow tint. Pass it through a fine-mesh sieve to remove possible lumps. Expert tip: Determine the readiness of the condensed milk by appearance and flavor. You will feel a slight caramel flavor. Also, the longer you cook the milk, the thicker, darker, and sweeter it will be.

Let it cool to room temperature. To prevent condensed milk from forming a layer on top, allow it to cool while stirring occasionally. Refrigerate for 12 hours to thicken the product. If necessary, remove the milk foam with a teaspoon.

If the cooled, condensed milk seems still liquid, add 2 oz. (60 g) of sugar and bring to cook again. Stir constantly until the milk begins to thicken. Turn it off immediately once it reaches the right consistency, and remove it from the heat. Wait for the final product to cool to room temperature and bring it to the fridge.