In large bowl, beat cake mix, bananas, oil, water and eggs with electric mixer on low speed 30 seconds. Beat on medium speed 2 minutes, scraping bowl occasionally until smooth. In bottom half of pan (without holes), fill each well with 1 level measuring tablespoon of cake batter. Place top half of pan on top and secure with keys. (Cover remaining cake with plastic wrap; place in refrigerator.) Bake 18 to 22 minutes or until toothpick inserted in center comes out clean. Cool 5 minutes in pan. Remove cake balls from pan; cool completely on wire rack. Repeat with remaining cake batter cleaning and spraying pan before filling with batter.