Preheat oven to 325 degrees. You will need a medium baking dish and a large baking pan to put the medium one in.
Pour 1 cup sugar in warm pan over medium heat. Constantly stir sugar until it’s brown and becomes caramel. Quickly pour the caramel in the medium baking dish, tilting it to swirl the caramel around the side, this must be done quickly since the caramel will start to harden immediately.
In a mixer or with a whisk, blend the eggs together. Mix in the milks then the vanilla. Blend smooth after each ingredient is added.
Pour custard into caramel lined baking dish. Fill the large baking pan with about 1-2 inches of hot water then place the custard filled dish in the large pan. Bake for 45 minutes in the water bath and check the center with knife. If knife comes out clean, it's ready.
Invert the flan onto a plate, the caramel sauce will flow over the custard. Let the flan cool in the refrigerator for 1 hour.