Start by rolling out your puff pastry sheet on a lightly floured surface. Gently press it into your tart molds, ensuring there are no air bubbles. Trim the excess pastry and prick the bottoms with a fork to prevent puffing during baking. Pro tip: Chill the molds in the fridge for 15 minutes before baking to ensure the pastry holds its shape.

In a saucepan, combine milk, cinnamon stick, and lemon zest over medium heat. Bring it to a gentle simmer, allowing the flavors to infuse. Meanwhile, whisk together egg yolks, sugar, and flour in a separate bowl. Once the milk mixture is ready, slowly pour it into the egg mixture, stirring constantly to avoid curdling. Return the mixture to the saucepan and cook until thickened, creating a luscious custard base.

Pour the custard filling into the prepared pastry shells, filling each one almost to the top. Bake in a preheated oven at 475°F (245°C) for 12-15 minutes, or until the edges are golden brown and the custard develops those signature caramelized spots. The aroma will be intoxicating, and your kitchen will feel like a cozy Portuguese café.