Beat butter at medium speed with an electric mixer until creamy. Gradually add 2 cups flour and 1/2 cup powdered sugar. Beat just until a smooth dough is formed. Press into a lightly greased, foil-lined 13- x 9-inch baking pan. Bake at 350° for 20 minutes or until lightly browned.
Meanwhile, whisk together eggs, remaining 1/3 cup flour, sugar, and next 5 ingredients in a large bowl; pour over baked crust. Bake at 350° for 25 minutes or until set. Let cool completely in pan; chill, if desired. Sprinkle with powdered sugar, and cut into bars. Garnish, if desired.
Coconut-Pineapple Bars: Prepare crust as directed. Omit orange zest and orange and lemon juices. Add 3/4 cup or 1 (6-ounce) can pineapple juice and 1/2 cup sweetened flaked coconut to egg mixture. Bake as directed. Garnish with toasted coconut, if desired.
Key Lime Bars: Prepare crust as directed. Omit orange zest and orange and lemon juices. Add 3/4 cup Key lime juice to egg mixture. Bake as directed. Garnish with lime zest, if desired.