For the base: Position a rack in the middle of the oven and preheat to 180°C. Put the butter, flour, cocoa, icing and granulated sugars, chocolate and 1/4 teaspoon salt in a food processor. Process until the mixture is slightly damp and holds together when pinched, about 1 minute.
Generously spray a 9- by 13-inch pan with cooking spray. Line the pan with aluminum foil leaving a 2-inch overhang on all four sides and spray the foil as well. Sprinkle the crust mixture into the prepared pan and press evenly along the bottom. Take care to seal up any cracks. Bake the base until it is no longer shiny and leaves only a slight indentation when pressed, 25 to 30 minutes. After removing the base, reduce the oven temperature to 160°C.
Meanwhile, make the orange filling: Whisk the eggs, egg yolks, granulated sugar, flour and pinch of salt together in a large liquid measuring cup or bowl until smooth. Whisk in the orange and lemon juices and zests. Pour the filling over the warm crust. Bake until the filling is just set, 35 to 40 minutes.
Cool the bars in the pan completely on a rack. When cool, refrigerate for 2 hours or up to overnight. Lift the bars out of the pan by using the foil. Trim the edges and cut into 24 bars. Dust with icing sugar and top with a strip of candied orange, if using, before serving.