In heavy saucepan, bring sugars to a boil over medium-high heat. If crystals form on the sides of the pan, cover the pan for 30 seconds or as long as 5 minutes, so steam rinses off the sides.
When the sugar begins to darken, swirl the pan to even the color. When sugar is the color of iced tea, immediately pour it into a tube pan, casserole or shallow, rounded bowl. Using pot holders, swirl to coat the dish or pan; set aside to harden.
Place rack in lower third of the oven and set temperature to 300° F. Have a large roasting pan ready for a water bath.
Empty the milks into a large mixing bowl. Beat in eggs, one at a time. Add vanilla extract. When the mixture is somewhat frothy, pour it into the prepared pan over the hardened caramel.
Cover the flan with buttered aluminum foil. Place it in the roasting pan and set in the oven. Fill with very hot tap water to within 1 inch of the top of the flan.
Bake 1 hour and 30 minutes. A knife inserted in the center should come out clean. If not, continue baking another 5 to 10 minutes, checking with the knife for doneness.
Remove from water bath. Cool 5 minutes.
Carefully invert flan onto a large, round platter with a rim. The liquefied caramel will fall out with the flan and pool all around it.
Serve by the wedge, warm or cold.Makes 4