Make the Pastry Crust:

1. Prepare the dough: In a food processor, pulse together flour, powdered sugar, and salt. Add cold butter cubes and pulse until mixture resembles coarse crumbs with some pea-sized butter pieces remaining. Add egg yolk and vanilla, pulse until just combined. If dough seems too dry and won’t hold together when pinched, add ice water one tablespoon at a time.
2. Chill the dough: Turn dough out onto a clean surface and gently knead just until it comes together. Shape into a flat disk, wrap tightly in plastic wrap, and refrigerate for at least 1 hour or up to 2 days.
3. Roll and fit the pastry: On a lightly floured surface, roll chilled dough into an 11-12 inch circle about ⅛-inch thick. Carefully transfer to your 9-inch tart pan, gently pressing into the bottom and up the sides. Trim excess dough by rolling your rolling pin across the top of the pan. Prick the bottom with a fork several times. Freeze for 15 minutes.
4. Blind bake: Preheat oven to 375°F (190°C). Line the frozen tart shell with parchment paper and fill with pie weights or dried beans. Bake for 20 minutes, then carefully remove parchment and weights. Continue baking for 8-10 minutes more until the crust is golden brown and fully cooked. Let cool completely. Reduce oven temperature to 300°F (150°C).

Make the Lemon Filling:

5. Zest and juice the lemons: Finely zest 2 tablespoons of lemon zest, being careful to avoid the bitter white pith. Juice enough lemons to get ¾ cup of fresh juice. Strain out any seeds.
6. Cook the lemon curd: In a medium heavy-bottomed saucepan, whisk together sugar, eggs, egg yolks, lemon juice, lemon zest, and a pinch of salt. Add the butter pieces. Cook over medium-low heat, whisking constantly, for 8-12 minutes until the mixture thickens enough to coat the back of a spoon (should reach about 170°F on a thermometer). Do not let it boil or the eggs will scramble.
7. Strain and finish: Remove from heat and immediately strain the curd through a fine-mesh sieve into a clean bowl to remove any bits of cooked egg and zest. Stir in the heavy cream until smooth and well combined.
8. Fill and bake: Pour the warm lemon filling into the cooled tart shell. Carefully transfer to the 300°F oven and bake for 15-20 minutes, just until the filling is set around the edges but still has a slight wobble in the center.
9. Cool completely: Remove from oven and let cool to room temperature on a wire rack, then refrigerate for at least 2 hours or until fully set before serving.
10. Serve: Carefully remove the tart from the pan by pushing up the removable bottom. Dust with powdered sugar if desired, slice with a sharp knife (wiping clean between cuts), and serve chilled or at cool room temperature.