Preheat your oven to its highest setting (around 250°C/480°F) and grease a 12-cup muffin tin lightly.
Roll out the All-Butter Puff Pastry into a log, slice into 12 even pieces, then press each piece firmly into the wells of the muffin tin so the layers stand up the sides; chill the lined tin in the fridge while you make the custard.
Make a simple syrup by combining the Granulated Sugar and Water in a small saucepan, bringing to a boil until the sugar dissolves and the mixture thickens slightly (2–3 minutes); remove from heat and set aside to cool a little.
Warm the Whole Milk gently in another saucepan with the Cinnamon Stick, Lemon Peel and a pinch of Salt until just below simmering to infuse the flavors; remove from heat and let infuse for 5 minutes, then discard the cinnamon stick and peel.
In a bowl whisk the Egg Yolks with the Plain Flour until smooth, then slowly whisk in the warm milk to avoid lumps; strain the mixture through a fine sieve into a clean bowl for an extra-smooth custard.
Gradually whisk the cooled sugar syrup into the strained custard to combine, tasting for sweetness and adjusting as desired.
Pour the custard into the chilled pastry cases, filling each about three-quarters full, and bake on the middle rack for 10–15 minutes until the pastry is deeply golden and the custard has blistered dark spots on top; watch closely near the end so it caramelizes but doesn't burn.
Allow the tarts to cool in the tin for a few minutes before removing to a wire rack. Just before serving, dust the tops with Icing Sugar and a light sprinkle of Ground Cinnamon.
