Scrape seeds from vanilla bean with tip of a sharp knife into a bowl. Whisk in flour, sugar, salt and zest. Blend in butter with fingertips or pastry blender until mixture resembles coarse meal with some small (roughly pea-sized) lumps. Drizzle with beaten yolk and gently stir with a fork until incorporated. (Do not overwork dough or it will become tough.)
Turn out dough onto a work surface and divide into 2 portions. Smear each portion once with the heal of your hand in a forward motion to distribute fat. Gather both portions of dough and form into a single disk, then chill, wrapped in plastic, until firm, at lest 30 minutes.
Roll out dough between sheets of waxed paper into an 11-inch round. Chill round until firm, about 10 minutes.
Remove top sheet of waxed paper. Invert dough onto tart pan and remove remaining paper. Fit dough into pan, then trim edge flush with rim. Lightly prick bottom and sides of shell all over with a fork. Chill until firm, about 15 minutes.
Preheat oven to 375.
Line shell with foil and fill with pie weights. Bake in middle of oven 20 minutes. Carefully remove weights and foil and bake until golden, about 15 minutes more. Cool shell completely in pan on a rack.
Whisk together milk, yolks, zest and juice until well blended. Pour into tart shell and chill until set, at least 8 hours. (Filling is not baked.)
Melt chocolate in top of a double boiler or in a metal bowl set over a pan of barely simmering water, stirring, until smooth. Spread into a 4-inch round on a waxed-paper-lined plate. Chill until firm, about 30 minutes. Remove chocolate from waxed paper and pull vegetable peeler across it to make thin curls. Chill curls on another sheet of waxed paper until firm, about 15 minutes.
Remove rim of pan and top tart with chocolate curls.