Preheat your oven to 425°F (220°C).
Pour the sweetened condensed milk into a shallow glass baking dish or pie plate.
Cover it tightly with aluminum foil.
Place that dish inside a larger baking pan and fill the outer pan with hot water until it reaches halfway up the sides of the inner dish.
Bake for 2 to 2.5 hours, checking every 45 minutes to add more water if needed.
When it’s thick and golden brown, give it a good whisk to smooth out the texture.
Tip: The longer you cook it, the deeper and richer the flavor.