Step 1: Prepare the Pastry - Preheat your oven to 220°C (428°F). Grease the muffin tin with oil. Roll out the puff pastry and cut circles for the muffin cups.
Step 2: Make the Custard Filling - In a saucepan, combine sugar and water. Boil for 3 minutes, then cool. In another saucepan, heat milk with cinnamon and lemon zest, then steep for 10 minutes.
Step 3: Whisk the Egg Yolks - In a bowl, whisk egg yolks and flour. Gradually add warm milk, then stir in syrup and vanilla.
Step 4: Fill the Pastry Shells - Pour custard into pastry shells, filling three-quarters full.
Step 5: Bake the Tarts - Bake for 15-20 minutes until golden. Watch carefully to avoid burning.
Step 6: Cool and Serve - Let cool slightly before transferring to a cooling rack. Serve warm, dusted with powdered sugar or cinnamon.