In a medium saucepan, combine the sugar, water, lemon zest, and cinnamon stick over high heat. Bring to a boil for 1 minute, then remove from the heat, and let sit until ready to use.
In a separate saucepan, heat 1 cup of milk over medium heat until bubbles begin to form around the edges.
In a large bowl, whisk the flour with 5 tablespoons of milk. Continue whisking while adding the hot milk in a slow stream. (discard the lemon zest and cinnamon stick)
Whisk the syrup into the milk mixture in a steady stream. Return to the saucepan and cook over low heat, whisking constantly, until thickened.
Add the yolks to the mixture and whisk until well combined, then strain through a fine-mesh sieve. Pour 1 ½ tablespoons of the warm filling into each pastry shell.
Bake until the shells are golden brown and crisp, the custards are set, and the tops are blackened in spots. Let cool in the pans on wire racks for 5 minutes. Then, remove the molds, transfer the tarts to the wire racks and sprinkle with cinnamon.
Serve warm and enjoy with family and friends!