Easy Chocolate Banana Cake

1 (18.25 ounce) box white or yellow cake mix
1/3 cup vegetable oil
4 eggs
8 ounces sour cream
8 tablespoons unsweetened cocoa powder
1/2 cup semisweet chocolate chips
1 large ripe banana, mashed

Preheat oven to 350 degrees F. Lightly grease and flour a 13 x 9-inch pan.

Mix together cake mix, oil, eggs, sour cream, cocoa, chocolate chips and banana. Beat for two minutes. Pour into prepared pan.

Bake for 35 minutes. Let cool in pan, then dust with confectioners' sugar, if desired.