Add the flour and salt into a medium-sized bowl and stir to mix well. Pour in the water. Mix with a spatula until the water has fully incorporated and a dough starts to form.

Lightly dust a clean work surface. Turn the dough onto the surface. Knead briefly, 5 minutes or so, until it forms a ball with a slightly coarse surface. Place the dough back into the bowl and cover with plastic wrap or a damp towel. Let rest for 15 minutes. The dough will become smooth and very sticky.

Dust the working surface with flour again. Turn the dough onto the surface. Gently shape the dough into a ball without kneading (to avoid developing gluten). Then roll the dough into a thin large rectangle sheet, about 1/8” thick.

Using a small offset spatula, spread 1/3 of the butter all over the surface of the dough, leaving a 1” border at the edges. It’s okay if there’s a few small tears at this step, or if the butter doesn’t spread perfectly evenly.

Fold the bottom third of the dough up to cover the middle third, and then fold the remaining top third down so it covers the first folded part. Rotate the dough 90°, adding more flour as necessary to avoid sticking.

Repeat rolling and folding process a second time. If at any point there is excess flour on the top of the dough before adding butter, brush off lightly with a pastry brush.

Turn the folded dough another 90°, and repeat this one more time with the last third of the butter.

With the long side of the dough facing you, roll the sheet once again into a rectangle, about 1/8” thick. Trim 1/2” from all edges to even the sides of the dough.

Roll dough away from you, into a log, and then gently shape it with your hands to even it out.

Cut the log in half with a sharp knife, and wrap both halves in plastic. Refrigerate for 2 hours, or freeze for 1 hour, until firm.

Combine the sugar, water, lemon peel, and cinnamon stick in a saucepan. Bring to boil for 1 minute, and remove from heat to cool. If using lemon extract and ground cinnamon, this can be added once the syrup is removed from heat.

In another medium-sized saucepan, add the flour, milk, and salt. Cook over medium heat, whisking constantly until thickened, 3 minutes or so. Set aside to cool for at least 10 minutes.

Once the flour milk paste mixture has cooled off, place a small strainer over the pot. Pour in the syrup over the strainer, then discard the lemon peels and cinnamon. Whisk to mix the syrup.

Add the egg yolk and whisk again, until evenly incorporated.

Place a small strainer over a large measuring cup or a bowl with a pouring spout. Pour the custard mixture through the strainer. Use a spatula to push any residual pieces through the strainer. You might want to repeat this step to get a more smooth and even mixture.

Preheat the oven to 425°F (218°C). Cut each dough log into 1 1/4” (3 cm) pieces, creating 10 pieces in total.

Dip the cut ends of pieces in flour to lightly coat, and with the cut side facing up. Press then roll it with a rolling pin to form an even circle, about 3” (7.5cm) inches in diameter. Firmly press the dough into an un-greased muffin tin, smooth and press it from the bottom to the top, until the dough reaches just over the top edges. Repeat with the remaining dough pieces.

Fill the pressed dough pieces about 3/4 full with the custard.

Bake at 425°F (218°C) for 15 to 20 minutes, rotating halfway through, until custard is set and pastry is light brown on edges. Turn the oven up to 500°F (260°C) and continue baking for 5 more minutes, until the pastry is golden brown and the custard has dark, blistered patches. Note, depending on the location of your oven’s heating element, the custard might not brown evenly. To achieve the best char result, you might need to use a blowtorch after baking the egg tarts.

Once the egg tarts are cooked, transfer the muffin tin to a cooling rack. Allow to cool a bit before removing the egg tarts. Enjoy the tarts still warm or at room temperature.

(Optional) You can sprinkle the egg tarts with a mixture of 1 tablespoon granulated sugar + 1 teaspoon ground cinnamon as garnish.

Store the egg tarts in an airtight container in the fridge for 3 to 4 days. Reheat in a 400°F (200°C) oven for 5 to 10 minutes, to slightly warm up the tarts before serving. Do not freeze the egg tarts.