Step 1: Prepare the Muffin Tin
Preheat your oven to 425°F (220°C). Generously brush a 12-cup muffin tin with melted butter, making sure to coat the bottom and sides of each cup thoroughly. This ensures your tarts release easily after baking.

Step 2: Shape the Pastry Shells
On a lightly floured surface, roll out the thawed puff pastry to about ⅛-inch thickness. Using a round cookie cutter or glass (about 4 inches in diameter), cut out 12 circles. Gently press each circle into a muffin cup, allowing the pastry to come up the sides naturally. The pastry should reach about ¾ of the way up the sides. Don’t stretch it too thin or it won’t hold the custard properly.

Step 3: Make the Custard Base
In a medium saucepan, combine the milk, heavy cream, and lemon peel (if using) over medium heat. Heat until the mixture is steaming and just beginning to simmer around the edges – don’t let it boil. Remove from heat and discard the lemon peel.

Step 4: Create the Sugar-Flour Mixture
In a mixing bowl, whisk together the sugar, flour (or cornstarch), and salt until well combined. This prevents lumps from forming in your custard.

Step 5: Temper the Egg Yolks
In a separate bowl, whisk the egg yolks until smooth. Gradually add the sugar-flour mixture to the yolks, whisking constantly until you have a thick, pale yellow paste.

Step 6: Combine and Cook the Custard
Slowly pour about ½ cup of the warm milk mixture into the egg mixture, whisking constantly. This tempers the eggs and prevents scrambling. Gradually whisk in the remaining milk mixture until completely smooth.

Step 7: Strain and Flavor
Pour the entire mixture back into the saucepan through a fine-mesh strainer to remove any lumps. Place back on medium-low heat and cook, stirring constantly with a whisk, for 3-5 minutes until the custard thickens slightly and coats the back of a spoon. Remove from heat and stir in vanilla extract.

Step 8: Chill the Custard
Transfer the custard to a bowl and press plastic wrap directly onto the surface to prevent a skin from forming. Refrigerate for at least 2 hours or until completely cold. This step is crucial – warm custard will make soggy pastry!

Step 9: Fill the Tarts
Once the custard is cold, give it a good stir. Fill each pastry shell about ¾ full with custard – they should be generously filled but not overflowing. The custard will puff slightly during baking.

Step 10: Bake to Perfection
Bake in the preheated oven for 20-25 minutes, until the pastry is golden brown and puffed, and the custard tops have developed those signature dark caramelized spots. The tarts should jiggle slightly in the center but be set around the edges.

Step 11: Cool and Serve
Remove from the oven and let cool in the pan for 5 minutes, then carefully transfer to a wire rack. The tarts are best served warm or at room temperature. Dust with cinnamon or powdered sugar just before serving if desired.