Heat oven to 350°F (325°F for dark or nonstick pan). Spray bottom and sides of 15x10x1-inch pan with baking spray with flour.
In large bowl, beat cake ingredients with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Pour into pan.
Bake 20 to 25 minutes (22 to 28 minutes for dark or nonstick pan) or until toothpick inserted in center comes out clean and cake springs back when lightly touched in center. Cool completely, about 1 hour.
In medium bowl, beat frosting ingredients on high speed until smooth and creamy; add more lemon juice if needed. Spread over cake. Store loosely covered.