Combine flour, salt, and cold water in a bowl. Mix until dough just comes together and pulls away from the sides.

Transfer dough onto a floured surface. Knead for 1-2 minutes to form a round. Cover and let rest for 15-20 minutes.

Roll dough into a square, dusting with flour as necessary, and spread 1/3 of the butter over 2/3 of the square. Fold it like a letter. Freeze for 10 minutes.

Repeat rolling, buttering, and folding process twice more; chill dough for at least 2 hours or overnight.

Make sugar syrup. Boil sugar, water, cinnamon, and lemon zest until it reaches 210-215°F. Remove from heat.

Cook custard base by whisking flour, salt, and milk over medium heat until thickened, about 5 minutes. Cool for at least 10 minutes.

Mix egg yolks, sugar syrup, and vanilla extract into cooled milk mixture. Strain custard into a glass container.

Preheat the oven to 550°F (288°C) and grease a 12-cup muffin tin.

Cut dough log into 12 pieces, press into muffin cups forming a tart shell that extends slightly past the rim.

Fill each tart shell 3/4 of the way with custard and bake for about 12 minutes, or until the pastry is browned and the tops caramelize.