Un-wrap puff pastry from package. Lightly flour puff pastry and flatten pastry with rolling pin. Cut puff pastry into four inch squares.
Place four inch square into muffin pan. Chill in fridge for 15 minutes as you make filling.
Preheat the oven to 500°.
In a medium saucepan, combine the sugar, water and cinnamon stick over high heat. Bring to a boil and cook for 1 minute, then remove from the heat and let sit until ready to use.
In a small saucepan, heat 1 cup of milk. In a large bowl whisk flour and remaining milk. Once milk in saucepan is hot slowly pour the hot milk into the large mixing bowl. Once combined, add the simple syrup into mixing bowl (discard cinnamon stick from sugar syrup). Whisk everything and return mixture to saucepan and and cook over low heat, whisking constantly, until thickened 10 to 12 minutes.
Add the egg yolks to the mixture and whisk until well combined, then strain through a fine-mesh sieve. Pour 1½ tablespoons of the warm filling into muffin pan.
Bake until the shells are golden brown and crisp, the custards are set, and the tops are blackened in spots, 15 to 20 minutes. Let cool for 5 minutes. Then, remove the molds, transfer the tarts to the wire racks and sprinkle with cinnamon. Serve warm.