Caramelize sugar - place sugar in small heavy saucepan. Heat over medium heat, watching carefully, until sugar is melted and turns deep golden brown (approximately 4-5 minutes, but cooking times will vary depending on heat and pan type). Immediately remove from heat and pour into 9-inch flan dish or deep-dish pie plate. Holding dish with potholders, quickly tilt dish to coat bottom completely and evenly. Syrup will harden quickly.
Heat oven to 350°F. Mix condensed and evaporated milks in medium saucepan; Heat until very hot. Milk should be steaming but not bubbling. Meanwhile beat eggs, vanilla and salt in medium bowl until blended but not foamy; slowly stir in hot milk. Optional: Strain egg mixture through a fine sieve to remove any lumps (this will provide for a smoother finished custard).
Place pan on rack in center of 350°F oven; pour very hot water into baking pan to within 1/2 inch of top of flan dish. Bake until knife inserted near center comes out clean, 35 to 40 minutes. Remove dish from water bath at once; cool on wire rack. Gently loosen edges with tip of knife. Invert onto platter. Serve warm or cold.