Preheat the oven to 175 degrees. Lightly grease two bread loaf tins.
In a large bowl, mix margarine, sugar and eggs together. Grate the bananas in the puree and add to the margarine and add the vanilla. Sift flour, soda and cocoa, mix well. Mix sour cream and chocolate. Pour the dough into prepared trays.
Bake in a preheated oven for 60 minutes.