In a small pot heat the sugar over medium heat until dissolves and turns a light brown color. It should begin to smell like caramel.
Remove from heat and pour the liquid into the ramekins, distributing it equally.
In a large mixing bowl stir together the egg yolks, sweetened condensed milk, evaporated milk and seeds from the vanilla bean. Stir until the ingredients are well combined. Pour into the ramekins.
Bake at 350°F in a water bath for 45 minutes. They will still have a slight jiggle even after baking.
Allow to fully cool and then refrigerate for at least an hour. I like to make flan the day before I plan to serve it so it can refrigerate overnight.
Run a thin knife around the edge of the flan, between the custard and the ramekin dish. Then place a small plate over the ramekin and flip over. The flan should easily remove from the ramekin, if not give it a few light taps to loosen the bottom.
Notes
Evaporated milk can be substituted for whole milk in this recipe.
I use 6 egg yolks because I love the richness and color it brings to this recipe. That being said you could use 3 eggs (whites + yolks) instead. But if you do use all yolks save the white to make meringue, egg white bites or omelettes.
Vanilla bean can be substituted for 1 teaspoon vanilla extract. But I really love using vanilla bean in this recipe as it adds that beautiful speckled look to the flan as well as so much flavor. Spanish flan is a very simple, made of very few ingredients, so using high quality vanilla bean elevates it.
Tips for baking with a water bath. I will place the ramekins in a casserole or roasting pan, whatever I plan to bake them in. Then fill with the caramel and custard batter. I heat water in my gooseneck kettle (to 200°F or around this) and then once I place the baking pan in the oven I carefully fill it with the hot water. Removing the pan from the oven once baking is done is the hardest part—just be careful not to spill the hot water.
Instead of 6 ramekins you could bake this in one, 9-inch glass pie pan for one large flan instead of individual ones. Increase the baking time by 5-8 minutes.