Heat oven to 350 degrees F. Line 24 standard-size muffin cups with foil or paper liners. Grate zest from lemon and squeeze juice (you should have 1/4 to 1/3 cup). Add water to juice to make 1-1/3 cups. Beat cake mix, egg whites, zest, juice and water at medium speed in large bowl. Increase speed to high; beat 2 to 3 minutes. Stir in lemon extract and poppy seeds. Fill muffin liners one-half to two-thirds full.
Bake in 350 degree F oven 22 to 24 minutes, until cakes springs back when pressed. Cool on rack.
Mix frosting, lemon juice and zest. Spread onto cupcakes. Cut thin wedges of lemon; pat dry. Coat with sugar. Press onto cupcakes. Makes 24 cupcakes.