Crust: Put the flour, confectioners' sugar, and salt in a food processor and pulse a couple of times to combine. Scatter the pieces of butter over the dry ingredients and pulse until the butter is coarsely cut in. Stir the yolk, just to break it up, and add it a little at a time, pulsing after each addition. When the yolk is in, process in long pulses—about 10 seconds each—until the dough, which will look granular soon after the egg is added, forms clumps and curds.
Turn the dough out onto a work surface and, very lightly and sparingly, knead the dough just to incorporate any dry ingredients that might have escaped mixing. This is done by smearing the butter pieces into the flour using the heel of your hand—a French technique called "fraisage."
Flatten dough into a disc and place between two pieces of parchment paper. Using a rolling pin, roll out the dough, rotating it ⅛ of a turn at a time and rolling until it's just bigger than the tart pan.
Butter the tart pan. Press the dough evenly over the bottom and up the sides of the pan, using all but one little piece of dough, which you should save in the refrigerator to patch any cracks after the crust is baked. Using a fork, give the crust a few pricks and freeze for at least 30 minutes before baking.
Parbake the crust: Preheat the oven to 375 degrees F. Butter the shiny side of a piece of aluminum foil and fit the foil tightly against the crust. Add pie weights on top of the foil to keep the crust flat as it bakes. Put the tart pan on a baking sheet and bake the crust for 25 minutes.
The filling: Cut the lemons into small pieces and remove the seeds. Put the lemons and sugar in a blender or food processor and pulse, blending and scraping down the sides until you have a smooth mix. Add the remaining filling ingredients and pulse and blend on low speed until the filling is homogeneous. Tap the bowl on the counter several times to "de-bubble" the filling as much as possible.
Remove the tart shell from the oven and carefully remove the foil and pie weights. Place back in the oven for 8–10 minutes to lightly brown. Remove the tart and patch any cracks with the leftover dough. Cool and reduce oven temperature to 325 degrees F.
Bake the tart: Stir the filling, then pour it into the tart shell. Bake for 20 minutes, then increase the oven temperature to 350 degrees F and bake for an additional 25–30 minutes. (Total baking time is 45–50 minutes.) When the tart is properly baked, it should be set, although perhaps still shaky in the center, and most of the top will have formed a light sugary crust. (Don't be alarmed if the tart has bubbled over the edge of the tart—that's okay.) After cooling the tart to room temperature, set the tart on top of a sturdy can so the ring can drop away. Place on a dish and serve.