HEAT oven to 350° F. Line cookie sheets with parchment paper.
BEAT cake mix, eggs, water and oil in medium bowl with an electric mixer on medium speed for 2 minutes, scraping bowl occasionally.
DROP batter onto prepared cookie sheets, preferably using a medium scoop (2 tablespoons). Spread batter into 2 1/4-inch diameter circles, 2 inches between pies. Bake 7 to 9 minutes or until center is set. Cool 1 minute on cookie sheet. Cool completely on cooling rack.
BEAT cream cheese in small bowl until light and fluffy with electric mixer on medium speed. Add frosting, mixing until blended. Place half of pies flat side up. Spread with 2 tablespoons filling in center of each. Top with remaining pies, rounded side up. Press gently to spread filling. Chill until ready to serve.
TIP Whoopie pies may be wrapped individually in plastic wrap and stored in the refrigerator for several days.