In a heavy saucepan, combine granulated sugar and ¼ cup of water over medium heat. Stir occasionally until the sugar dissolves, then let it boil undisturbed until it turns a golden brown, about 10 minutes. Immediately pour the caramel into a flan mold, swirling it to coat the bottom before it hardens.

In a mixing bowl, whisk together the eggs until fully blended. Gradually incorporate the sweetened condensed milk, whole milk, and vanilla extract, stirring gently. A smooth consistency is key for your Spanish Flan.

Carefully pour the custard into the prepared flan mold over the hardened caramel. Cover the mold tightly with aluminum foil. Place the flan mold inside a larger baking pan and fill the outer pan with hot water until it reaches halfway up the sides of the flan mold.

Preheat your oven to 350°F (175°C). Bake for 1 hour to 1 hour and 15 minutes, until the flan is set but still slightly jiggly in the center.

Once baked, remove the flan from the oven and let it cool at room temperature for about 30 minutes. Refrigerate for at least 4 hours, then run a knife around the edges, place a serving plate upside down over the mold, and flip to reveal the flan.