Preheat the oven to 350 degrees F. In a saucepan, combine the milk, sugar and lemon zest and bring to a boil. In a medium-sized bowl, combine 3 egg yolks, the vanilla, and the cornstarch and whisk well to combine. Add the hot milk in a stream, whisking constantly. Return this mixture to the pan and cook over a low flame for 10 minutes, stirring constantly.

Roll out the pastry dough, reserving a piece the size of a baby's fist. Line an 8-inch tart pan with the dough. Cover the bottom with half the cooked cream mixture, then spread the jam over that. Add the remaining cream mixture. Roll out the reserved dough to form a top for the tart, and gently lay it over the tart, pinching the edges. Brush the top with the remaining egg yolk and bake 45 minutes. Let cool, then sprinkle with confectioners' sugar.

In a medium bowl, combine the flour, salt, and sugar. Cut the butter into the dry ingredients until the mixture resembles coarse crumbs. In a small bowl, combine the egg, the yolk, and the vanilla, and stir this mixture into the flour mixture with a fork until it just comes together. Turn the mixture onto a floured board and gently work the mixture until a dough is formed. Press the dough into a disc shape about 4 inches across, wrap tightly in plastic wrap and freeze for 15 minutes.