Preheat oven to 350 degrees. Line an 8-by-8-inch glass baking dish with nonstick foil that overhangs the sides (this "sling" makes removing the finished bars cleaner and easier).
Make the crust by combining the flour, sugar, and salt. Combine the vanilla, ½ teaspoon zest, and melted butter, then stir into flour mixture to make a dense dough. Press evenly and firmly in bottom of prepared baking dish. Bake 16 to 20 minutes, until deep golden brown. Remove and reduce oven to 325 degrees.
Meanwhile, make the filling by whisking together the sugar, starch, baking powder, and salt in a medium saucepan. Whisk in the juice and eggs. Over medium heat, stir constantly until the mixture boils and thickens (a thick layer of it will cling to the spoon), 4 to 8 minutes. Once thickened, continue to cook, stirring vigorously, for about 30 more seconds, then remove from heat and stir in the zest and butter until combined. Strain if desired to remove lumps, then spread evenly over crust.
Bake for 8 to 12 minutes, until the lemon mixture is just set (do not overbake). Allow to cool to lukewarm before transferring to refrigerator to chill completely. Note that the filling will remain very soft and lemon-curd like.
When ready to serve, slice into squares or rectangular bars. Dust with confectioners' sugar. For an optional garnish, top each bar with a fresh raspberry.
To freeze, separate individual bars uncovered on a tray and freeze solid, then wrap tightly and return to freezer. Thaw loosely covered in the refrigerator. Garnish with sugar and berry when ready to serve.