Preheat the oven to 350-degrees. Grease an 9x9x2-in. pan with cooking spray or softened butter. Line with parchment paper and set aside.
Add the flour, salt and sugars to a large mixing bowl and whisk to combine. Add the melted butter and stir with a rubber spatula. Transfer to the prepared pan and press down with your fingers (or the bottom of a measuring cup) until compact and flat. Poke the dough all over with a fork and place in the freezer for at least 30 minutes to chill.
Bake the crust for 18 to 20 minutes, until golden brown, rotating at the halfway point.
Meanwhile, make the filling. Wipe the large mixing bowl with a paper towel, add the sugars and zest. Whisk to combine and then using your fingers, rub the zest into the sugars. Add the flour and salt and whisk to combine. Add the eggs and yolk, and whisk gently until smooth. You do not want to add extra air into the filling or the tops of the bars will crack while baking. Add the juice and gently whisk a final time.
Pour the filling over the crust and bake for 18 to 20 minutes, until the filling has set, and still jiggles just slightly in the center. Do not over bake tor the top might crack. Cool to room temperature, and then refrigerate for about an hour, or until cold, before slicing and serving with a dusting of confectioners’ sugar.