Set over moderately low heat, place the sugar and allow to melt and caramelize to a rich golden brown (this will take about 40 minutes). Do not stir the sugar as it melts but occasionally shake the skillet. Add the boiling water, teaspoon by teaspoon at first, stirring briskly to dissolve the caramelized sugar. Simmer uncovered 8 to 10 minutes until the consistency of maple syrup. Pour into a chilled, well-buttered, shallow 2 quart
For the flan: Preheat the oven to moderately slow (325 degrees). In a large heavy saucepan, combine the milk, heavy cream and sugar; drop in the orange zest and bring to a simmer over moderately low heat, stirring from time to time. Beat the egg yolks and eggs until frothy; blend 1 cup of the hot cream mixture into the eggs, stir back into the pan, and heat, stirring constantly, 1 minute. Remove from the heat and mix in the vanilla.