Make the Pastry Dough: In a bowl, mix 205 g flour with 4 g salt. Add 132 ml water and mix vigorously until a dough forms. Let rest for 30 minutes.
Roll and Layer Butter: Roll dough into a thin rectangle. Spread softened butter (~200 g) evenly over the dough. Make several slices in the dough, then fold corners over the middle repeatedly to create layers. Wrap in plastic and chill for 15 minutes.
Roll and Shape Pastry Log: Flour the board and roll dough thin again. Trim edges to form a neat rectangle. Spread remaining butter over dough. Roll tightly from the long side into a log. Pinch seam and wrap in plastic. Chill for at least 2 hours (up to 8 hours for best flavor).
Prepare Sugar Syrup: In a pot, combine 130 g sugar, 80 ml water, half a vanilla bean (scraped and skin), 1 cinnamon stick, lemon peel slices, and a bit of lemon juice. Heat over medium without stirring for 5-10 minutes until thick and sticky. Stir briefly and set aside to cool and thicken.
Make Custard Base: In a new pot, whisk 40 g flour, 350 ml milk, and a pinch of salt. Cook over medium heat, stirring continuously until thickened and pale yellow. Remove from heat.
Temper Egg Yolks: Whisk 6 egg yolks separately. Slowly add some hot milk mixture to yolks while stirring, then pour yolk mixture back into pot. Add cooled sugar syrup while whisking.
Strain and Chill Custard: Strain custard through a fine sieve to remove lumps. Cover with plastic wrap touching the surface to prevent skin. Refrigerate for at least 2 hours, preferably 8 hours.
Preheat Oven: One hour before baking, preheat oven to highest setting (around 260°C / 500°F) using top element only, no fan or convection.
Prepare Pastry Cups: Cut chilled pastry log into 1-2 cm thick discs. Press each disc into aluminum tart molds, pushing dough up the sides to form cups.
Fill and Bake: Fill pastry cups about two-thirds full with custard. Place molds on a tray and bake for 10-15 minutes. Activate bottom oven element midway to ensure even cooking and blistered tops.
Cool and Serve: Remove tarts from oven and cool for 15 minutes before serving. Optionally sprinkle with cinnamon before eating.