Position an oven rack in the upper third of the oven and heat the oven to 450°F (230°C). Generously grease a standard 12-cup muffin pan with butter or nonstick cooking spray.

On a lightly floured surface, unfold the thawed puff pastry sheets. Use a 4-inch (10-cm) round cutter to cut 12 circles, rerolling scraps as needed. Gently press one pastry circle into each muffin cup, allowing the pastry to come slightly above the rim. Chill the pan while preparing the custard.

Pour 1/2 cup (120 ml) of the milk into a small bowl. Whisk in the flour until completely smooth, with no lumps.

In a medium saucepan, combine the remaining 1 cup (240 ml) milk, heavy cream, sugar, lemon peel, cinnamon stick, and salt. Warm over medium heat, stirring occasionally, until steaming and the sugar has dissolved; do not let it boil rapidly.

Remove the lemon peel and cinnamon stick. Whisk the flour-milk mixture once more, then slowly whisk it into the warm milk mixture. Cook over medium heat, whisking constantly, until the custard thickens enough to lightly coat the whisk, about 2 to 3 minutes. Remove from the heat.

Let the custard cool for 5 minutes. In a separate bowl, whisk the egg yolks and vanilla. Slowly whisk about 1/2 cup (120 ml) of the warm custard into the yolks, then whisk the yolk mixture back into the saucepan. For the smoothest filling, strain the custard through a fine-mesh sieve into a measuring cup with a spout.

Fill each pastry-lined muffin cup about three-quarters full. Bake until the pastry is deeply golden and the custard has browned spots and a gentle jiggle in the center, 15 to 18 minutes. If the pastry browns before the custard develops spots, move the pan one rack higher for the final few minutes.

Cool the tarts in the pan for 10 minutes, then carefully loosen them with a thin knife and transfer to a rack. Serve warm or at room temperature, with a light dusting of ground cinnamon and powdered sugar if desired.