Preheat the oven to 180C/160Fan/350F. Grease and line a deep loaf tin In a large bowl, sift together the flour, bicarbonate of soda and cocoa powder. Set aside. In a large bowl, cream together the butter and sugar until light and fluffy. Gradually add the beaten eggs, beating well after each addition. Beat in the vanilla. Stir in the mashed bananas, walnuts or pecans if using, chocolate chips and sour cream. Fold in the dry ingredients until just combined. Quickly pour the mixture into the lined loaf tin. Sprinkle the remaining 25g chocolate chips over the top and bake for 60-80 minutes or until a skewer inserted in the middle of the cake comes out clean. Leave to cool completely in the tin.