1. Heat oven to 350 degrees. Grease bottom only of 13x9-inch pan with shortening and lightly flour, or spray with baking spray with flour.
2. In medium bowl, mix pretzels, sugar and butter. Sprinkle evenly on bottom of pan; press gently.
3. In large bowl, beat cake mix, margarita mix, water, oil, grated lime peel and egg whites with electric mixer on low speed 30 seconds. Beat on medium speed 2 minutes, scraping bowl occasionally. Pour over pretzel mixture.
4. Bake 34 to 39 minutes or until light golden brown and top springs back when touched lightly in center. Cool completely, about 2 hours.
5. Whip heavy cream until stiff peaks. Frost cake; sprinkle with grated lime peel. Store covered in refrigerator.