Preheat oven to 350. Place parchment paper on 2 cookie sheets
Using an electric mixer, cream butter and sugar in medium bowl until light and fluffy. Add egg whites and stir until just blended. Sift cake flour over butter mixture and mix with spoon. Fold in vanilla
Fold in the zest.
Drop heaping teaspoons of batter onto prepared cookie sheets leaving at least 3-inches between each cookie. When cookie sheet is full flatten out cookies with back of spoon so cookies look almost translucent.
Bake until edges start to brown, about 4 or 5 minutes. Remove from oven and quickly transfer cookies to a cooling rack using a thin metal spatula. Yields four cookies per serving..For a nutty flavored cookie, skip the zest and add almond extract.
only makes 10 cookies..I usually double the recipe.