The classic utensil for melting the sugar on top of a flan is a salamander, which is first heated in coals or on a burner.
Dissolve the cornstarch in 1/2 cup of the milk. Beat together granulated sugar and egg yolks until pale yellow and thick, then beat in cornstarch mixture. Set aside.
In a medium saucepan over low heat, combine remaining 3 cups of milk, the lemon zest, and the cinnamon stick. Heat to boiling. Whisking quickly, add the egg-cornstarch mixture; lower heat and continue to stir until thickened. Strain, then pour into shallow dessert dishes or small ramekins to a depth of 5/8 inch. Let cool.
Sprinkle flans with superfine sugar. With a hot salamander, or under a broiler, heat tops until golden brown.