Position a rack in the center of the oven and preheat to 180 C. Line a 9x13 glass baking dish with foil leaving a 2-inch overhang on two sides. Grease the foil and sides of the dish with butter.

To make the crust, pulse the zest with 1/3 cup of the sugar in a food processor until the sugar is yellow and sandy, 1 to 2 minutes. Add 1 1/2 cups of the flour, the butter, cornstarch and salt and pulse until the dough comes together, about 1 minute. Press the dough evenly into the bottom of the prepared pan. Bake until the crust is golden at the edges and pale golden in the center, about 30 minutes. Transfer the dish to a wire rack.

Reduce the oven temperature to 150 C. To make the filling, whisk the eggs and egg yolks with the remaining 1 2/3 cups sugar and the remaining 1/2 cup flour in a bowl until smooth. Whisk in 1 cup lemon juice.

Pour the filling over the warm crust and return to the oven. Bake until the filling is just set, about 30 minutes.

Cool the lemon bars on a wire rack, 30 minutes, then refrigerate uncovered until firm, at least 1 hour. Lift the bars out of the pan using the foil. Peel off the foil and cut into 24 bars. Dust with confectioners' sugar. Place a piece of gummy lemon candy on top of each square and serve.