1 1/2 cups coarsley crushed pretzels
1/2 cup sugar
1/2 cup butter or margarine, melted
1 pkg. white cake mix
1 1/4 cups bottled non-alcoholic margarita mix
1/3 cup vegetable oil
1 tablespoon grated lime zest
3 egg whites
1 (8 oz.) Cool Whip
Heat oven to 350. Grease bottom only of 9x13 pan; lightly flour.
Mix pretzels, sugar and butter in medium bowl. Sprinkle evenly on bottom of pan; press gently.
Beat cake mix, margarita mix, oil, lime zest (use only green part, avoiding the white, bitter part) and egg whites in large bowl with electric mixer until fluffy, about 2 minutes or so. pour batter over pretzel mixture.
Bake 35 to 40 minutes or till light brown and top springs back when touched lightly in center. Cool completley, about 2 hours.
Frost with whipped topping. If desired, sprinkle with more lime zest.
Store covered in refrigerator.