HEAT oven to 350ºF.
BEAT cake mix, butter and 2 eggs with mixer on low speed 30 sec., stopping frequently to scrape beater and side of bowl. Beat on medium speed 2 min. Spread onto bottom of 13x9-inch pan sprayed with cooking spray.
BEAT remaining ingredients until well blended; pour over batter in pan.
BAKE 30 min. or until edges are firm. Cool slightly.