Preheat oven to 500°F. Lightly grease a 12-cup muffin pan with butter. Stack both sheets of pastry on top of each other, then roll into a tight log from one end. Cut the log into 12 (about 1/2-inch-thick) slices. Dip fingertips in water; press the center of each slice down with your fingertips to flatten it. Dip thumbs in water, and gently stretch and pull pastry to form a 3-inch wide shallow bowl. Press each pastry bowl into the bottom and up the sides of each muffin cup extending the edges about 1/4-inch above the top of pan. Leave at room temperature until ready to bake.
Place milk, butter, cinnamon stick, and lemon peel in a medium saucepan over medium.
While milk mixture heats, whisk together sugar, cornstarch, and flour in a medium bowl until combined. Place egg yolks in a large bowl, and whisk until smooth.
When the milk mixture is just beginning to boil, use a slotted spoon to remove and discard the cinnamon and lemon peel. Whisk in the flour mixture, whisking constantly, until thickened but not boiling, about 1 minute. Remove from heat. Gradually add milk-flour mixture to the egg yolks, whisking constantly, until well combined. Strain through a sieve into a heatproof measuring cup with spout.
Divide custard evenly among prepared pastry in muffin cups (about 3 tablespoons per cup).
Bake in preheated oven until dough is golden and the filling is puffed, about 12 minutes. Without removing pastries from oven, increase oven temperature to broil. Broil until filling is browned in spots, 2 to 3 minutes. Remove from oven, let cool slightly, about 5 minutes. Remove from pan; cool completely on a wire rack.
