For crust: Mix first 8 ingredients in processor. Using on/off turns, cut in butter until mixture resembles coarse meal. Blend in egg yolk and and just enough milk to bind dough; do not form ball. Gather dough together; flatten to disc. Wrap in waxed paper and plastic and refrigerate at keast 3 hours. (Can be prepared 1 day ahead.).
Very lightly butter 10-inch tart pan with removable sides. Roll pastry out between sheets of waxed paper to 1/8-inch thick round. Transfer to prepared pan, discarding paper and pressing gently into bottom and up sides, Run rolling pin across top to trim.(Can be prepared 1 day ahead to this point.).
Position rack in lower third of oven and preheat to 400. Pierce bottom of crust with fork. Line with parchment or foil and fill with pie weights or dry beans. Bake 12 minutes. Remove weights and paper and continue baking until crust is light brown, about 7 minutes. Cool on rack. Reduce oven temperature to 350.
For filling: Using electruc mixer, cream butter until light and fluffy. Add 1 cup sugar, 1/3 at a time, and beat until light and fluffy. Blend in corn starch and salt. Mix in eggs one at a time, then mix in yolks one at a time. Stir in fresh lemon juice, grated lemon peel and vanilla; mixture may separate,.
Pour filling into crust. Bake until top is pale golden brown and custard is set, about 30 minutes. Cool completely on rack. Dust top with powdered sugar. Arrange candied zest around edge, or lemon slices in center. Serve with whipped cream if desired.