In an electric mixer, with the paddle attachment, cream together butter and sugar until light, Add egg whites, extract and zest and beat until blended well. Combine dry ingredients and add to the mixing bowl on low speed. Mix until almost blended. Remove bowl from mixer and stir gently to finish blending. With a 2 fluid ounce scoop (1/4 cup), scoop the dough onto a parchment-lined cookie sheet and flatten slightly with damp fingers. Sprinkle tops of cookies with a generous amount of granulated sugar. Bake 18 to19 minutes, until just beginning to brown around the edges. Let cool on cookie sheet 5 minutes, then remove cookies to a wire rack to cool completely.