Preheat oven to 350 degrees.
In a medium saucepan over medium-low heat, melt sugar until liquefied and golden in color. Carefully pour hot syrup into a 9-inch round glass baking dish, turning the dish to evenly coat the bottom and sides. Set aside until completely cool.
In a large bowl, beat eggs. Mix in condensed milk, evaporated milk and vanilla until smooth. Pour egg mixture into baking dish. Cover tightly with aluminum foil.
Bake in hot water-bath in a preheated oven for 60 to 70 minutes maximum. Check if flan is done by inserting a clean knife in the center and if the knife comes out half clean and half watery that's fine since the flan will harden in the fridge when left overnight. It's very important to let flan cool at room temperature for 4 to 5 hours and then place in fridge overnight.
To serve, carefully invert on serving plate after warming the baking dish in a hot water bath.