Roll out the puff pastry and cut into 12 circles to fit into a muffin tin. Press the pastry into the tin and set aside.
In a saucepan, combine sugar and water. Bring to a boil and let it simmer for about 3 minutes. Remove from heat.
In another saucepan, whisk together milk and flour. Add the cinnamon stick and lemon peel. Cook over medium heat until thickened, about 5 minutes. Remove from heat and discard the cinnamon stick and lemon peel.
Whisk the egg yolks and vanilla extract into the milk mixture. Slowly add the sugar syrup, whisking constantly.
Pour the custard into the pastry-lined muffin tin. Bake for 20-25 minutes or until the tops are browned and the custard is set.
Let the tarts cool in the tin for a few minutes before transferring to a wire rack to cool completely.