Place parchment paper on 2 cookie sheets. (You may need to make more batches depending on the size of your cookie sheets).
Using an electric mixer, cream butter and sugar in a medium bowl until light and fluffy. Add egg whites and stir until just blended. Sift cake flour over butter mixture and mix with a spoon; fold in vanilla.
Fold lemon and lime zest into bowl.
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 a spoon so cookies look almost translucent and measure about 3-inches across.
Bake until edges start to brown, about 4 to 5 minutes. Remove from oven and quickly transfer to a cooling rack using a thin metal spatula.
Yields approximately 40 cookies. Store cookies in an airtight container to retain crispness.
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