Step 1

Mix 1 3/4 cups all-purpose flour, 2/3 cup water, and 1/4 teaspoon kosher salt together in a large bowl with your hands until a shaggy dough forms. Knead until the dough is elastic and smooth but still a little tacky, 5 to 8 minutes. Shape the dough into a ball, wrap in plastic wrap, and let rest at room temperature for 20 minutes.

Step 2

The dough should now look very smooth. Generously flour a work surface, rolling pin, and the dough. Roll the dough out into an 8x10-inch rectangle (about 1 centimeter thick) with a long side closer to you.

Step 3

Brush any excess flour off the dough. Spread 3 pieces of the unsalted butter in a thin layer onto the left 2/3 of the rectangle. Lift up the right unbuttered portion and fold it over the middle section, then fold the far left panel over the middle.

Step 4

Rotate the dough 90 degrees. Roll the dough out again into an 8x10-inch rectangle. Repeat buttering, folding, rotating, and rolling out the dough two times more. Spread the remaining butter on the dough, leaving a 1/2-inch border.

Step 5

Starting with the long side closest to you, tightly roll the dough up into a log. Trim both ends, then cut the log in half crosswise. Wrap each one in plastic wrap. Refrigerate for 30 minutes. Meanwhile, make the custard filling.

Step 6

Using a vegetable peeler, peel 2 long strips zest from 1 medium lemon, being careful not to peel off any of the white pith. Place the zest, 1 1/3 cups granulated sugar, 2/3 cup water, and 1 (3-inch) cinnamon stick in a small saucepan. Bring to a boil over medium-high heat. Simmer, swirling pan occasionally but not stirring, until thickened to the consistency of syrup, 2 to 3 minutes. Measure out 3/4 cup plus 2 tablespoons of the syrup and set aside.

Step 7

Whisk 1/3 cup of the whole milk and 1/4 cup of all-purpose flour together in a medium bowl until smooth. Bring the remaining 1 cup whole milk to a boil in a medium saucepan over high heat. Remove the saucepan from the heat.

Step 8

While whisking constantly, slowly pour half of the hot milk into the flour mixture. Pour the mixture back into the saucepan. Add the measured syrup and cook over medium heat, stirring constantly, until thickened and smooth, 3 to 5 minutes. Remove the saucepan from the heat.

Step 9

Place 5 large egg yolks in the now-empty milk-flour bowl and whisk to combine. Temper the eggs by slowly whisking in the remaining hot milk. Transfer to the saucepan and whisk to combine. Push the filling through a fine-mesh strainer set over a bowl or liquid measuring cup; discard the contents of the strainer.

Step 10

Arrange a rack in the top third of the oven and heat the oven to 550°F. If your oven doesn’t heat up to 550°F, place a pizza stone or 2 stacked baking sheets on the rack while the oven heats.

Step 11

Cut 1 of the dough logs crosswise into 12 pieces. (If your kitchen is warm, use one piece at a time and keep the remaining refrigerated.) Place a piece cut-side down in an egg tart tin or a well in a standard 12-well muffin tin. Use your thumb to firmly press the center of the dough into the bottom of the tin, forming a wall of dough around your thumb. If using egg tart tins, press the edges of the dough outward and up against the sides until it extends over the lip of the tin by 1/4 inch. If using a muffin tin, push the dough up against the sides of the well, but the dough will not reach the top. Make sure the dough is as thin as you can get it. If the dough is sticking to you, dip your thumb in water before shaping.

Step 12

Repeat until you have shaped all 12 pieces. Place the dough-lined tins on a baking sheet. Fill the tins or wells with the custard, stopping 1/4 inch from the top.

Step 13

Bake until the custard is slightly puffed and browned in spots and the crust is golden-brown and looks flaky, 10 to 16 minutes. Let cool for 10 minutes, then carefully unmold them onto a wire rack.

Step 14

Repeat assembling and baking the remaining dough log and custard into 12 more egg tarts, letting the tins or muffin tin cool completely if reusing them.

Step 15

Dust the egg tarts with a little powdered sugar and ground cinnamon right before serving if desired.