In a small saucepan, combine sugar, water, cinnamon stick, and lemon peel. Bring to a simmer over medium heat until sugar dissolves. Remove from heat and let infuse.

In another saucepan, whisk flour with a little milk to make a paste. Whisk in remaining milk. Cook over medium heat, whisking, until thickened (5-7 mins). Remove aromatics from syrup and whisk hot syrup into the milk mixture. Let cool for 10-15 minutes.

In a bowl, whisk egg yolks and vanilla. Slowly whisk in a few spoonfuls of the warm milk mixture to temper. Pour the yolk mixture back into the saucepan and whisk until fully combined. Strain the custard through a fine-mesh sieve.

Preheat oven to 550°F (290°C). Tightly roll the puff pastry sheet into a log and cut into 12 equal pieces. Place each piece cut-side up in a muffin cup. Using your thumb, press the dough down and up the sides to form a thin shell.

Fill each pastry shell about three-quarters full with the custard. Bake on the upper rack for 12-17 minutes, until the pastry is golden and the tops have dark, caramelized spots.

Let cool in the muffin tin for a few minutes before transferring to a wire rack. Serve warm, dusted with cinnamon and powdered sugar if desired.