Combine 1 cup of granulated sugar and 1/4 cup of water over medium heat. Stir gently until boiling and then let it turn golden brown, about 8-10 minutes. Pour the caramel into a round pan, tilting to coat evenly before it hardens.

In a large mixing bowl, whisk together 4 eggs, 1 can of sweetened condensed milk, 1 1/4 cups of whole milk, and 1 tablespoon of vanilla extract until just combined.

Pour the custard mixture into the caramel-coated pan. Cover tightly with aluminum foil. Place the pan inside a larger dish filled with warm water to create a bain marie.

Preheat the oven to 350°F (175°C). Place the bain marie setup in the oven and bake for 1 hour and 15 minutes until the flan is set around the edges and slightly jiggly in the center.

Let the flan cool at room temperature for about 30 minutes. Refrigerate for at least 4 hours, ideally overnight to fully set.

Run a knife around the edges of the flan to loosen it. Invert a serving platter over the pan and flip it upside down to release the flan.