1️⃣ Prepare the caramel:

1. In a saucepan over medium heat, dissolve the sugar in water.
2. Stir occasionally until it turns into a golden caramel (about 7-10 minutes).
3. Immediately pour the caramel into a round baking mold, tilting to coat the bottom evenly. Set aside.

2️⃣ Make the custard:

1. In a blender, combine condensed milk, evaporated milk, eggs, vanilla extract, and rum (if using). Blend until smooth.
2. Pour the custard mixture over the caramel in the mold.

3️⃣ Bake the quesillo:

1. Preheat oven to 350°F (175°C).
2. Place the mold inside a larger baking dish filled with hot water (bain-marie method).
3. Bake for 60 minutes or until a toothpick inserted in the center comes out clean.

4️⃣ Chill and serve:

1. Remove from the oven and let it cool to room temperature.
2. Refrigerate for at least 3 hours or overnight for the best texture.
3. To serve, run a knife around the edges and invert onto a plate to reveal the caramel topping.