Preheat oven to 350 degrees F (175 degrees C). Lightly grease two 9x5 inch loaf pans.
In a large bowl, cream together margarine, sugar and eggs. Stir in bananas and vanilla. Sift in flour, baking soda and cocoa; mix well. Blend in sour cream and chocolate chips. Pour batter into prepared pans.
Bake in preheated oven for 60 minutes, or until a toothpick inserted into center of a loaf comes out clean.