Twelve 3 1/4-inch brioche a tete molds; a candy thermometer; a pastry bag fitted with a star tip; kitchen torch, optional
For the crust: Place 12 identical 3 1/4-inch brioche a tete molds in the freezer.
Combine flour and salt in the bowl of a food processor and pulse to 2 to 4 times, then add butter and pulse 8 to 10 times, until texture looks mealy and no large lumps of butter remain. Pour on water and pulse 5 times. (At this point, the mixture should hold together when squeezed.) Turn the dough out onto a sheet of plastic wrap and draw the plastic up around it, squeezing and pressing the dough into a ball. Flatten into a 1-inch-thick disk and refrigerate for 15 minutes.
Remove the dough from the refrigerator, unwrap and place on a lightly floured piece of parchment paper. Dust the top of the disk with a small amount of flour and top with a second sheet of parchment. Roll dough out into a round 1/8-inch thick. Use a 5-inch biscuit cutter and cut 6 circles out of the dough. Re-roll once if necessary.
Remove the brioche molds from the freezer. Line 6 of the molds with the dough, being careful to press the dough into all the nooks and crannies. Gently run the rolling pin over the tops of the molds to trim the edges. Nestle each of the empty brioche molds inside the molds with the dough. Place the sandwiched molds in the freezer refrigerator for 15 minutes.
Meanwhile, preheat the oven to 300 degrees F and position a rack in the center of the oven with a second just beneath it.
Remove the sandwiched molds from the freezer. Place them, inverted, directly on the center rack with a sheet pan on the rack below to catch any butter that might render out. Bake for 30 minutes. Remove the molds from the oven. Flip right-side up and remove the top molds.
Turn the oven up to 400 degrees F, return the tart shells to the oven and bake until golden brown, about 10 minutes more. Remove the tart shells from the oven and let cool completely on a baking rack before removing the shells from the molds.
For the lemon filling: Whisk egg yolks until light in color in a medium nonreactive bowl and set aside.
Whisk cornstarch, sugar, water and salt in a medium saucepan and bring to a simmer over medium heat. Remove from heat and temper the yolks by slowly adding the hot starch mixture to them in a steady stream, whisking constantly, until a third of the starch mixture has been worked into the eggs, then return that mixture to the saucepan. Whisk in the butter and continue heating until just boiling.
Stir in the lemon juice and zest, reduce heat to medium-low and whisk constantly until mixture returns to a boil. Cook for 1 full minute more, then transfer to a medium bowl over an ice bath. Cool, stirring occasionally, until the curd is just slightly warmer than room temperature. (Do not chill or cool completely, or the curd will become difficult to spread.) Pour or use a pastry or zip top bag with the corner cut off to pipe the mostly cooled curd into the tart shells until flush with the tops of the shells. Cool filled tarts completely on a baking rack while you make the meringue topping.
For the meringue: Whisk together the egg whites until foamy, then add the sugar, cream of tartar and salt in the heat-proof bowl of a stand mixer. Place the bowl over a pot of simmering water and whisk constantly until the mixture reaches 165 degrees F. Then return the bowl to the base of the stand mixer and whip the egg whites on medium-high speed until medium-stiff peaks form. Increase the speed to high and whip until stiff peaks form, about 5 minutes total. Transfer the meringue to a pastry bag fitted with a star tip.
Pipe the meringue to cover the top of the cooled tarts. Brown the meringue under a preheated low broiler, about 1 minute, or with a kitchen torch.
Propane gas torches are highly flammable and should be kept away from heat, open flame and prolonged exposure to sunlight. They should be used only in well-ventilated areas. Follow torch manufacturer's instructions for use.