Prepare the Caramel:

• In a nonstick pan, melt the granulated sugar over medium-low heat, stirring constantly until it turns a golden amber color.
• Quickly pour the caramel into a 7-inch round flan mold, tilting it to coat the bottom evenly. Set aside to harden.

 Make the Custard:

• In a blender, combine eggs, sweetened condensed milk, whole milk, vanilla extract, and optional rum. Blend for 30–60 seconds until smooth.

 Assemble for Baking:

• Pour the custard mixture over the caramel layer in the mold. Cover the mold tightly with foil.
• Place the mold in a large baking dish and fill the dish with hot water halfway up the sides of the mold.

 Bake the Quesillo:

• Preheat the oven to 375°F (190°C).
• Bake for 60–90 minutes, or until a knife inserted in the center comes out clean with a slight jiggle.

 Cool and Set:

• Remove the quesillo from the water bath and let it cool at room temperature for 30 minutes.
• Refrigerate for at least 4–8 hours, ideally overnight, to fully set.

 Unmold and Serve:

• To unmold, run a knife around the edges of the flan.
• Dip the bottom of the mold in warm water for 1–2 minutes to loosen the caramel.
• Invert onto a serving plate and let the caramel drizzle over the flan.