In a small saucepan, combine sugar with 1/4 c. water. Bring to a simmer over low heat while stirring slowly. Increase heat to medium-high and continue cooking without stirring, until the syrup turns a deep amber color, do not allow too darken too much or the syrup will taste burned!
Swirl the pan if the syrup is coloring unevenly. Immediately pour the syrup into a 1 1/2 qt. souffle dish or casserole and carefully tilt the dish so that the caramel syrup coats halfway up the side of the dish as well as the bottom.
In a large bowl, whisk together eggs and egg whites. Add condensed milk, skim milk and vanilla, blending well.
Pour the mixture through a fine strainer into the caramel-coated dish. Set the dish uin a larger shallow pan, such as a roasting pan. Pour enough hot water into the larger pan so it comes halfway up the side of the custard dish.
Bake for 60-70 minutes, or until the custard is set around the edges but still wobbly in the center.
Remove the dish carefully from it's water bath to a rack to cool to room temperature. Then cover with =plastic= wrap (not foil) and refrigerate for at least 4 hours or overnight.
To serve: Loosen flan around the edges with a knife or spatula, invert the flan onto a servingplate.