In a food processor, pulse together the flour, sugar, and salt until combined. Cut in the butter until crumbly. Add egg yolk, cold water, and vanilla, and pulse until clumps form.

Gather the dough into a ball, flatten into a disk, wrap in plastic, and refrigerate for about 1 hour.

Roll out the dough on a floured surface to ¼ inch thick, transfer to the tart shell, trim excess, and chill for another 30 minutes.

Preheat oven to 375°F (190°C). Prick the crust and line with parchment and weights. Bake for 15 min then remove weights and bake for another 5 min.

In a double boiler, whisk together the egg yolks, sugar, lemon juice, zest, and sea salt until thickened, about 8–10 minutes.

Remove from heat, add butter and chocolate, and stir until smooth and creamy.

Pour filling into the baked crust and bake for 10-12 minutes until just set.

Cool at room temperature, then refrigerate for at least 4 hours before serving.