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1
For the base: Position a rack in the middle of the oven and preheat to 350 degrees F. Put the butter, flour, cocoa, confectioners' and granulated sugars, chocolate and 1/4 teaspoon salt in a food processor.
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2
Process until the mixture is slightly damp and holds together when pinched, about 1 minute.
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3
Generously spray a 9- by 13-inch pan with cooking spray.
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4
Line the pan with aluminum foil leaving a 2-inch overhang on all four sides and spray the foil as well.
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5
Sprinkle the crust mixture into the prepared pan and press evenly along the bottom.
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6
Take care to seal up any cracks.
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7
Bake the base until it is no longer shiny and leaves only a slight indentation when pressed, 25 to 30 minutes.
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8
After removing the base, reduce the oven temperature to 300 degrees F.
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9
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.
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10
Whisk in the orange and lemon juices and zests.
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11
Pour the filling over the warm crust.
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12
Bake until the filling is just set, 35 to 40 minutes.
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13
Cool the bars in the pan completely on a rack.
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14
When cool, refrigerate for 2 hours or up to overnight.
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15
Lift the bars out of the pan by using the foil.
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16
Trim the edges and cut into 24 bars.
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17
Dust with confectioners' sugar and top with a strip of candied orange, if using, before serving.
