Preheat oven to 350° F. Line cupcake pans with paper baking cups.
Beat cake mix, water, fat-free sour cream, and egg whites in a 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.
