Make the crust: In the bowl of a stand mixer fitted with the paddle attachment or in a large bowl using an electric mixer, beat the butter and confectioners' sugar at medium speed to combine. Scrape down the bowl, add the egg yolk, and beat until well blended. Scrape down the bowl and add half of the flour; beat on low speed until crumbly. Stop the machine, add the remaining flour and the cream, and beat on low speed just until a dough forms.
Turn out the dough and shape it into a ball, then flatten the ball slightly into a disk. Wrap in plastic wrap and refrigerate until firm, about 2 hours or up to 1 day.
Dust a work surface with flour. Cut the chilled dough into 1-inch pieces; using the heel of your hand, knead the pieces back together into a smooth disk. Keeping the surface well floured, shape the disk into a 12-inch-long log. Cut into 6 equal pieces. Refrigerate 10 minutes.
Remove one of the dough pieces and transfer back to the well-floured work surface. Using your fist, squish the dough into a disk, then roll using a rolling pin to form a very thin (1⁄8-inch) round. Center the round over a 4 3⁄4-inch fluted tart pan, preferably with a removable bottom, or a tart ring. Run your fingers around the inside of the pan or ring several times, easing the dough into the edges to avoid tearing. Be sure the dough fits against all the sides and nooks of the pan. Trim excess dough from the top edges. Dock the bottom of the dough sparingly with the tines of a fork. Transfer to a baking sheet and freeze for at least 10 minutes or until firm. Repeat with the remaining pieces of dough, refrigerating the scraps as you work in case they are needed to patch any holes.
Position a rack in the center of the oven and preheat to 375°. Transfer the baking sheet to the oven and bake until the pastry is light golden brown and the interior is dry, 12–15 minutes. If the bottoms of the shells puff up as they bake, tap them down lightly with your fingers or the back of a fork as often as needed. Remove and let cool in the tart pans completely.
Using a large spoon, fill the tart shells with the lemon curd. Refrigerate until set, about 30 minutes.
Make the meringue: In a standing mixer fitted with the whisk attachment or in a large bowl with a handheld electric mixer, whisk the egg whites on medium speed for 1 minute. Add the cream of tartar, increase the speed to medium-high, and mix for 1 minute. With the motor running, gradually add the sugar in small (1–2 teaspoon) increments. Once all the sugar has been added, continue beating the egg whites until stiff, glossy peaks have formed.
Carefully unmold the tarts. Place a generous dollop of meringue in the center of each. Caramelize the meringue with a kitchen torch if desired. Serve immediately, or refrigerate up to 1 hour. Let sit at room temperature for 10 minutes before serving.