Preheat the oven to 375º F (190º C). Place oven rack in the middle.
Make the Caramel:
Add the sugar in a heavy-bottomed pot or pan over medium-low heat, constantly stirring to prevent any burnt spots. If your caramel looks very lumpy and grainy, don’t worry. Just lower the heat and keep stirring until sugar dissolves and syrup turns deep amber.
Very carefully pour caramel into the flan mold. Using oven mitts as aid, immediately tilt the mold to coat sides. Set aside and let it cool down for about 10 minutes, or until the caramel hardens.
Make the Custard:
Add eggs, sweetened condensed milk, whole milk, and vanilla into a blender. Blend for 2 -3 minutes, or until everything is well combined.
Pour the custard into the caramel-coated flan mold. Cover tightly with the lid or aluminum foil.
Prepare the Water Bath:
To bake the quesillo, we are going to use a water bath, a.k.a Bain-Marie. Fill a large baking pan with about an inch or so of hot water (not boiling). The baking pan needs to be larger than the flan mold. Ideally, you want something that’s just a couple of inches larger than the flan mold.
Place the covered flan mold with custard in the center of the baking pan with the hot water. Carefully transfer the baking pan to the middle oven rack, and bake for 60 minutes. The flan will be ready when the center is just set (mixture will jiggle). Set aside and let it cool at room temperature. Then transfer to the refrigerator and chill for at least 4 hours before serving.
To unmold, run a knife around edges and invert onto a large rimmed serving platter.