Lightly flour your work surface. Unfold puff pastry, roll into a tight log, wrap in plastic, and chill for 30 minutes.

Cut the chilled log into 12 ½-inch (1.25 cm) discs. Place each disc in a muffin cup. Using a wet thumb, press the dough from the center up the sides to form a thin shell. Freeze the tin for 15 minutes.

In a saucepan, simmer sugar, water, cinnamon stick, and lemon peel until sugar dissolves. Let infuse off the heat.

In a bowl, whisk milk, flour, cornstarch, and salt. Remove aromatics from syrup and slowly whisk hot syrup into the milk mixture. Return to saucepan and cook over medium heat, whisking constantly, until thickened to a thin pudding consistency (5-7 minutes). Cool for 10 minutes.

Preheat oven to 550°F (290°C). Temper egg yolks by whisking in a small amount of the warm custard base, then whisk the yolk mixture and vanilla into the main custard batch.

Strain the custard through a fine-mesh sieve. Fill each chilled pastry shell three-quarters full. Bake for 12-15 minutes until the pastry is golden and the tops are blistered and caramelized. Cool slightly before serving.