Portuguese Custard Tarts with Cinnamon (Pastéis de Nata)

Roll out the thawed puff pastry on a lightly floured surface into a rectangle about 1/8 inch thick. Roll it tightly into a log, then cut the log into 12 equal slices. Place each slice cut-side up into a muffin tin or specific Pastéis de Nata molds. Using your thumbs, press the pastry from the center outwards to form a thin cup, ensuring the bottom is very thin and the sides are slightly higher than the mold.

In a medium saucepan, combine 1 cup of granulated sugar, 1/2 cup of water, the cinnamon stick, and lemon peel. Bring to a boil over medium heat, stirring until the sugar dissolves. Boil for 3 minutes without stirring, then remove from heat and let cool slightly. Remove the cinnamon stick and lemon peel.

In another saucepan, whisk together 1 1/2 cups of whole milk and 2 tablespoons of all-purpose flour until smooth. Cook over medium heat, stirring constantly, until the mixture thickens, about 3-5 minutes. Remove from heat.

Gradually whisk the warm sugar syrup into the thickened milk mixture. In a separate bowl, lightly whisk the 6 large egg yolks. Slowly temper the egg yolks by gradually whisking a small amount of the warm milk mixture into the yolks, then pour the tempered yolks back into the main milk mixture, whisking constantly until fully combined. Strain the custard through a fine-mesh sieve to ensure a smooth consistency.

Pour the custard mixture into the prepared pastry shells, filling each about 3/4 full. Bake in an oven at 500°F/260°C for 15-20 minutes, or until the pastry is golden brown and the custard is caramelized with dark spots on top. The high heat is crucial for the characteristic blistered top.

Remove the tarts from the oven and let them cool in the molds for a few minutes before transferring them to a wire rack to cool completely. Dust with 1/2 teaspoon of ground cinnamon before serving.