Butter two cupcake trays, each with six molds. Roll out one sheet of puff pastry and cut it into six pieces. Repeat the process with the second sheet. Gently press the puff pastry pieces into the cupcake molds, ensuring they cover both the base and sides.

In a saucepan, combine sugar and flour. Gradually pour in the milk, stirring continuously. Add the lemon peel. Place the saucepan over medium heat and bring to a near boil. Once the milk is warm, remove the lemon peel.

In a separate bowl, whisk the egg yolks thoroughly. Slowly pour half of the warm milk mixture into the yolks, whisking constantly to prevent the yolks from curdling. Transfer the yolk and milk mixture back to the saucepan with the remaining milk. Continue stirring over medium heat until the mixture thickens. Remove from heat and stir in the vanilla extract.

Strain the custard mixture to ensure a smooth texture. Pour the custard into the puff pastry-lined molds, filling each about 3/4 full. Bake in a preheated oven at 220°C (428°F) for 15-20 minutes or until the tops are golden brown and slightly blistered.

Once baked, allow the tarts to cool. Before serving, sprinkle with icing sugar