In a mixer or large bowl, with a whisk, add the sugar and flour, mixing well. Add the lemon zest and juice, orange zest and juice, beer, eggs, egg yolks and salt. Mix on high speed for 3 – 4 minutes, until the mixture is light and bubbly. Pour this egg mixture over the cooked shortbread crust. Place into the center of the oven and bake for another 28 – 35 minutes. The bar cookie will be done when the egg mixture is set, with a light crust atop, but not browned.