Process the flour, sugar, and salt in a food processor for a few seconds until combined. If you don’t have a food processor, you can do this by using a pastry cutter. Add butter and pulse until mixture becomes crumbly and resembles coarse meal, about 15 pulses. Add egg and vanilla extract and keep pulsing until the dough is no longer dry and starts to clump together, about 10-15 seconds. Do not process to the point that a large ball of dough is formed; rather the dough should be quite crumbly with large clumps.
Turn the dough to a lightly floured surface and form into a ball. It should come together easily without being sticky. Flatten the ball slightly with your hands to form a thick disc. Wrap with plastic wrap and refrigerate for at least 1 hour.
If you don’t want to use the dough right away, you can refrigerate it for up to 3 days.
To roll out the dough: Take dough out of the fridge and let it sit on the counter for a few minutes to soften slightly for easy rolling. On a lightly floured surface, roll the dough out into an 11-inch circle, then place gently into a 9-inch tart pan; you can do this by flouring a rolling pan and rolling the dough loosely around it, then unrolling it into the pan. Brush away any excess flour on the surface. With a sharp knife, trim the edges of the pastry to fit the tart pan. Cover the pan and place in the freezer until firm, about 30 minutes. Frozen dough is less prone to shrinking while baking.
To bake the crust: Preheat the oven to 190 C and place the rack in the centre.
Press parchment paper or aluminium foil tightly against the crust, covering the edges to prevent them from burning. Fill with uncooked pasta or rice, making sure they’re fully distributed over the entire surface.
Bake pastry for 20 minutes, until foil no longer sticks to the dough. Transfer pastry to a wire rack and remove weights and foil.
To partially bake the crust: Bake for 5 minutes longer, it will then be ready for your tart filling.
For the Filling:
In a medium heatproof bowl, place eggs, sugar, lemon zest,lemon juice, and heavy cream, if using, and whisk to combine. Place the bowl over a saucepan of simmering water (bain-marie). Cook on medium heat, whisking constantly, until mixture becomes thick (ours was ready in 12 minutes, but it can take up to 20). It should coat the back of a wooden spoon when thick enough. The curd will thicken more once cooled.
Remove from heat and immediately strain mixture through a sieve. Add butter, a few cubes at a time, and whisk until completely melted and incorporated, and mixture is smooth. Take your time with it—the whisking makes for an airy and light texture. Allow to cool to room temperature before filling the tart.
Fill your tart pastry with the lemon curd, then refrigerate for at least 4 hours until chilled. Enjoy!