To Make the Crust:
In a food processor, pulse together the flour, sugar, and salt until combined. Add the cold butter and pulse until the mixture resembles coarse crumbs.
Add the egg and vanilla (if using) and continue pulsing until the dough just starts to come together, but don’t overmix. The dough should be crumbly with large clumps.
Turn the dough out onto a floured surface, form it into a ball, and flatten it into a disc. Wrap in plastic wrap and refrigerate for at least 1 hour.
Roll out the chilled dough into an 11-inch circle and place it in a tart pan with a removable bottom. Press the dough into the edges and trim any excess. Freeze the tart shell for 30 minutes.
Preheat the oven to 375°F (190°C). Line the crust with parchment paper and fill with pie weights or dried beans. Bake for 20 minutes, then remove the weights and bake for another 5-10 minutes until golden brown. Let the crust cool completely.
To Make the Lemon Curd:
In a heatproof bowl, whisk together the eggs, sugar, lemon zest, lemon juice, and heavy cream (if using).
Place the bowl over a saucepan of simmering water (bain-marie) and whisk constantly until the mixture thickens, about 10-20 minutes, or until it coats the back of a spoon and leaves a clear path when you run your finger through it.
Remove the bowl from heat and strain the lemon curd through a fine mesh sieve. Gradually whisk in the butter cubes until fully melted and the curd is smooth and silky.
Let the lemon curd cool to room temperature before filling the tart shell.
To Assemble the Tart:
Pour the cooled lemon curd into the fully baked tart shell and spread evenly.
Refrigerate the tart for at least 4 hours, or until the lemon curd is fully set.
Serve chilled, optionally garnished with fresh berries and whipped cream.