1Heat oven to 350°F (325°F for dark or nonstick pan). Grease and flour 13x9-inch pan, or spray with baking spray with flour. In medium bowl, mix all streusel ingredients except 1/2 cup chocolate chips. Stir in chocolate chips. Set aside.
2In small bowl, toss 1/2 cup chocolate chips with 1 tablespoon of the dry cake mix. In large bowl, mix bananas, sour cream and eggs; beat with electric mixer on medium speed until well blended. Add remaining cake mix, water and oil; beat on low speed until combined. Beat 2 minutes on medium speed. Stir in coated chocolate chips; spoon into pan. Sprinkle with streusel.
3Bake 35 to 44 minutes or until toothpick inserted in center comes out clean and top is light golden brown. Cool completely, about 1 1/2 hours. Store loosely covered.