In a medium bowl, combine 1 cup flour, salt, and cold water. Mix with a wooden spoon until the dough comes together and pulls away from the sides of the bowl.

Turn the dough out onto a well-floured surface. Knead for about 1 minute, then shape into a square. Cover with a clean kitchen towel and let rest for 15 minutes.

Roll the dough into a 12-inch square. Spread ⅓ of the softened butter over the dough, leaving a 1-inch border. Fold the dough into thirds like a letter, then fold into thirds again to form a small square. Cover and refrigerate for 15 minutes.

Place the dough with a folded edge facing you. Roll into a 12-inch square again and spread with another ⅓ of the butter. Fold into thirds, then thirds again. Cover and chill for 15 minutes. Repeat once more with the remaining butter.

In a small saucepan, combine sugar, ⅔ cup water, cinnamon stick, and lemon peel. Bring to a boil, stirring until sugar dissolves. Remove from heat and let cool completely.

In a large bowl, whisk together egg yolks, milk, and ¼ cup flour until smooth. Gradually whisk in the cooled sugar syrup (strained to remove cinnamon and lemon peel). Add vanilla extract and mix well.

Roll the chilled dough into a log about 12 inches long. Cut into 12 equal pieces. Place each piece in a muffin cup, with the cut side facing up. Dip your thumb in cold water and press the dough against the bottom and sides of the cup, creating a thin, even layer with a small lip at the top.

Preheat your oven to 500°F (260°C) or as high as your oven will go. Fill each tart shell about ¾ full with the custard mixture. Bake for 12-15 minutes, until the pastry is golden and the tops are caramelized with dark spots.

Allow the tarts to cool in the pan for 5 minutes, then carefully remove to a wire rack. Dust with ground cinnamon and serve warm or at room temperature.