Preheat your oven to 500°F (260°C) or as high as it will go. Lightly grease a 12-cup standard muffin tin with cooking spray or a thin coat of butter.

Unroll the thawed puff pastry sheet on a lightly floured surface and roll it tightly into a cylinder or log shape. Cut the log crosswise into 12 equal pieces, each about 1-inch thick.

Place one piece of pastry cut-side down into each muffin cup. Using your thumbs, press the dough from the center outwards and up the sides to create a thin pastry shell that reaches the top edge of the cup, then place the tin in the refrigerator to chill.

In a medium saucepan, combine the granulated sugar, water, cinnamon stick, and lemon zest. Bring to a boil over medium-high heat without stirring, and let it cook for about 3 to 4 minutes until it forms a light syrup.

In a separate small bowl, whisk together 1/4 cup of the milk and the flour until completely smooth. In a second saucepan, bring the remaining 3/4 cup of milk to a gentle simmer over medium heat.

Remove the simmering milk from the heat and whisk in the flour mixture until thickened. Carefully remove the cinnamon stick and lemon zest from the hot sugar syrup, then slowly pour the syrup in a thin stream into the milk mixture, whisking constantly until smooth.

Whisk the egg yolks and vanilla extract in a small bowl. Gradually whisk a ladle of the warm milk mixture into the yolks to temper them, then pour the yolk mixture back into the main pot, whisking vigorously to combine the custard.

Pour the warm custard filling into the chilled pastry shells, filling each about 3/4 of the way to the top. Bake for 15 to 20 minutes, or until the pastry edges are deeply golden brown and the custard is puffed with dark, blistered spots on top.

Remove from the oven and let the tarts cool in the pan for 5 minutes before carefully transferring them to a wire rack. Serve the tarts slightly warm, dusted with a pinch of ground cinnamon for an authentic bakery finish.