Preheat oven to 350 degrees. Line cupcake pans with paper baking cups. Beat cake mix, water, fat free sour cream, and egg whites or egg substitute in large bowl on low speed for 30 seconds. Increase speed to medium speed and continue beating for 2 minutes, scraping bowl occasionally. Divide batter between the 24 cupcake cups. Bake for about 17 minutes or until toothpick inserted in center comes out clean. Cool completely. Dust the tops of the cupcakes with powdered sugar using a sifter, sugar shaker, or wire mesh strainer.