Heat oven to 350°F (or 325°F for dark or nonstick pan). Spray bottom and sides of 15x10x1-inch pan with baking spray with flour.

2. In large bowl, beat cake mix, buttermilk, oil and eggs with electric mixer on low speed 30 seconds. Beat on medium speed 2 minutes, scraping bowl occasionally. Pour into pan.

3. Bake 15 to 20 minutes or until toothpick inserted in center comes out clean. Cool completely, about 1 hour.

4. In medium bowl, beat shortening, butter, grated lemon peel, lemon juice and powdered sugar on high speed until smooth and creamy; add more lemon juice if needed. Spread over cake.