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 4 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.