Preheat oven to 350°. In a bowl, cream butter and sugar. Add eggs, bananas, milk and vanilla. Combine the flour, cocoa, baking soda and salt; add to the banana mixture and mix just until combined. Fold in nuts if desired. Transfer to a greased 9x5-in. loaf pan.
Bake for 60-65 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before removing from pan to a wire rack.