To make condensed milk according to the recipe, take a pot with a thick bottom and pour in the milk, cornstarch, and vanilla powder. Mix quickly and coldly with a whisk to avoid the formation of lumps.
Then add the sugar and place the pot over medium-high heat. Wait for it to boil, then immediately move the pot to a much lower flame to allow the cornstarch to thicken the milk.
Stir occasionally with a wooden spoon for a few minutes, and when we obtain a milk that sticks to the sides of the pot while stirring, it's ready. Wait one minute before removing it from the heat.
At this point, the condensed milk is just made, so it's not thickened enough because it's still hot. As it cools, it will thicken. We can let the condensed milk cool down and use it immediately (within a maximum of 3 days) in recipes or store it in vacuum-sealed jars. After a couple of hours of natural cooling, it should be perfect for use.
To preserve the condensed milk, pour it while still boiling into glass jars (previously sterilized in boiling water). Let it thicken at room temperature in closed jars.