Place 1 cup of sugar in a flan mold (tube pan) and heat over medium flame until it melts and turns golden brown, stirring occasionally to prevent lumps. Spread the caramel up the sides of the pan with a spoon.
In a blender, blend the sweetened condensed milk, whole milk, two eggs, and one egg yolk until smooth.
Strain the milk mixture through a sieve, then pour it into the caramel-coated flan mold.
Place the flan mold in a bain-marie (water bath) and bake at 355°F (180°C) for 90–120 minutes, or until set.
Let the flan cool to room temperature, then refrigerate for at least 2.5 hours before unmolding. (For best texture and flavor, overnight chilling is ideal.)
To unmold, run a thin knife around the edges of the flan, place a serving plate on top of the mold, and carefully invert it. The caramel will flow over the flan. Serve chilled.
