Roll the pastry into a tight log from a short edge and chill for 20 minutes.
Cut into 12 rounds. Press each round, spiral-side up, into the muffin cups, thinning the pastry up the sides. Chill while making the custard.
Make the Custard:
Whisk the flour with 1/4 cup milk until smooth. Heat the remaining milk with the cinnamon and lemon peel until steaming, then whisk it into the flour mixture.
Boil the sugar and water for 3 minutes without stirring. Slowly whisk the syrup into the milk mixture.
Cool for 10 minutes, remove the cinnamon and peel, then whisk in the yolks and vanilla. Strain.
Bake:
Heat the oven to 500 F (260 C). Fill each pastry shell about three-quarters full.
Bake for 15 to 20 minutes until the pastry is deeply golden and the custard is blistered. Cool for 10 minutes before removing from the pan.
Notes
Keep the pastry cold for distinct flaky layers.
Do not fill the shells to the rim.
A hot oven creates the characteristic browned custard spots.