Line a loaf pan with saran wrap and set aside. To assemble, blend each flavor separately adding the frozen bananas first and blending until crumbly and no large pieces remain. In the strawberry flavor blend the strawberries with the bananas. Add the vanilla and milk and blend until smooth adding the cocoa powder in the chocolate layer at this point. Pour the first layer in the bottom and spread out till flat and smooth out as best you can. Put in the freezer for at least 30 minutes. Prepare the next layer and pour over top, spreading evenly again. Repeat for the final layer.