Begin by pouring 1 cup of granulated sugar and 1/4 cup of water into a medium saucepan. Heat over medium flame, stirring gently until the sugar dissolves. Continue cooking without stirring until it turns a rich golden brown, about 8-10 minutes. Quickly pour the caramel into a 7-inch round pan, swirling to coat the bottom evenly. Allow it to cool and harden while you prepare the custard.

In a large mixing bowl, gently whisk together 4 large eggs, 1 can of sweetened condensed milk, 1 cup of whole milk, and 1 teaspoon of vanilla extract. Be careful to mix just until combined to avoid creating bubbles. Once combined, gently pour the custard over the cooled caramel layer in the prepared pan.

Preheat your oven to 350°F (175°C) while setting up the bain-marie for the flan. Cover the custard-filled pan tightly with aluminum foil to prevent any water from entering. Place this pan inside a larger baking dish and carefully fill it with warm water until it reaches halfway up the sides of the smaller pan.

Transfer the bain-marie setup into the preheated oven and bake for approximately 1 hour and 15 minutes. The flan should still have a slight jiggle in the center when done.

Once baked, carefully remove the pan from the oven and the water bath, allowing it to cool on the counter for about 30 minutes. Afterward, place the flan in the refrigerator and let it chill for at least 4 hours, or ideally overnight, to fully set.

When you're ready to serve, take the flan out of the fridge and run a knife around the edges to loosen it from the pan. Carefully invert the flan onto a serving platter.