In a medium saucepan, combine 1 cup of granulated sugar, ½ cup of water, a cinnamon stick, and a 3-inch strip of lemon peel. Heat over medium heat, stirring gently until the sugar is completely dissolved, about 1 minute. Remove from heat to let flavors infuse while working on the custard.

In a separate bowl, whisk 3 tablespoons of all-purpose flour with a small amount of the 1¼ cups of whole milk to form a paste. Gradually add the remaining milk while continuing to whisk. Pour into a saucepan over medium heat and cook for 5-7 minutes, stirring constantly until thickened. Combine with the syrup and cool for 10-15 minutes until warm.

In a clean bowl, whisk together 6 large egg yolks and 1 teaspoon of vanilla extract. Slowly add the warm milk and syrup mixture to the egg yolks, whisking continuously to prevent curdling. Strain the custard through a fine-mesh sieve into another bowl for a silky texture.

Take the all-butter puff pastry and roll it into a log shape. Slice into 12 equal pieces. Grease a muffin tin and press each piece into the cups, forming small shells that rise above the edge. Ensure the pastry is chilled to keep its flaky consistency.

Preheat your oven to 550°F (290°C). Carefully pour the custard mixture into each pastry shell, filling them about three-quarters full. Place the muffin tin in the oven, baking for 12-17 minutes, or until the tops are beautifully golden with dark caramelized spots.

Once baked, remove the muffin tin from the oven and allow the Pastel de Nata to cool for a few minutes. Gently transfer them to a wire rack to cool completely. Optionally dust with powdered sugar and a sprinkle of cinnamon before serving.