• Preheat the oven to 350 F (180 degrees C).

For the caramel:

• Place a quesillera** or flan mold on the stove over low heat, and add the sugar and water. Stir the mixture with a wooden spoon until the sugar is completely dissolved.
• Remove the caramel from the heat when it thickens and gets that golden amber color.
• You can also make the caramel in a saucepan and then pour it into a round baking pan if you don`t have a flan mold.

For the quesillo or flan mixture:

• Using a blender or an electric mixer (or a whisk), combine the condensed milk, the eggs, the milk, the vanilla extract, and the optional rum/brandy in the blender or in a bow. Mix or blend until you have a smooth consistency.
• Pour the quesillo mixture into the quesillera or flan mold coated with the caramel. Cover with the lid if using the quesillera. If you are using a regular round baking mold, cover it with aluminum foil.
• Place the flan mold inside a larger baking tray that has water up to the halfway point to make a water bath for baking the flan.
• Bake for an hour or until you introduce a toothpick or skewer in the center of the quesillo/flan, and it comes out clean. Make sure you have enough water in the large baking tray while baking the flan.
• When the quesillo is ready, remove the mold from the oven and let it rest at room temperature for at least 30 minutes. Once it has cooled down, place it inside the refrigerator for a few hours before serving.
• To unmold and serve the quesillo, first use a small spatula or butter knife to loosen the edges. Then, gently flip it onto a serving plate and cut the quesillo into slices.