Whisk in flour, sugar, salt and zest.  Blend in butter with your fingertips or a pastry blender until mixture resembles coarse meal with some small 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.  Kneed lightly, once or twice to incorporate the butter.  Gather both portions of dough and form into a single disk and chill.

Preheat oven to 375 degrees.

Roll out dough between sheets of wax paper into an 11-inch round.  Chill round until firm, about 10 minutes.

Remove top sheet of wax paper. Place dough into a 9" tart pan with a removable bottom. Remove remaining paper.  Fit dough into pan, then trim edge flushw ith rim.  Lightly prick bottom and sides of shell all over with a fork.  Chill until firm, about 15 minutes.

Line shell with foil and fill with pie weights or beans.    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.

Filling
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).

Chocolate Curls Garnish (Optional) 
Melt chocolate in top of a double boiler of barely simmering water, stirring, until smooth.  Spread into a 4 inch round until firm, about 30 minutes.  Remove chocolate from wax paper and pull vegetable peeler across it to make think curls.  Chill curls on another sheet of wax paper until firm, about 15 minutes.

Remove rim of pan and top tart with chocolate curls.