The caramel: Heat the sugar for the caramel in a small, heavy frying pan over low heat until it begins to dissolve. Shake the pan slightly (do not stir) until all the sugar has melted. Increase the flame and let the sugar bubble and color. Pour the caramel into the mold and quickly turn it around in all directions, tipping it up in a circular motion until the surface - bottom and 2 inches up the sides - has been lightly coated with the caramel. If the caramel thickens and becomes sluggish, gently heat the mold in a pan of hot water or over low heat, depending on the material, and continue the coating action. Set aside to cool.
Put the milk, salt, sugar, and cinnamon or vanilla into a saucepan and bring slowly to a boil, stirring until the sugar has dissolved. Continue boiling slowly, taking care that it does not boil over, until the milk has reduced by about 2/3 cup. Set aside to cool.
Place an oven rack on the lowest rung of the oven and heat to 325 F.
Beat the eggs and yolks together and stir into the tepid milk. Pour the mixture through a strainer into the flan mold and place it in a hot water bath in the oven. Test after 2 hours with a skewer or cake tester; if it comes out quite clean, the flan is cooked. Remove from the oven, but allow to sit in the water bath for about 15 minutes longer. Remove and set aside to cool completely before refrigerating.