Because of the dark color, it can be tricky to know when the chocolate banana bread is done.  Test the banana bread by gently poking it in the center of the loaf.  If the top springs right back then it is fully cooked.  If it leaves and indentation then it needs to cook longer.  At that point, you should check it every few minutes. You can also test the banana bread by feeling the top of the loaf.  If it starts to feel like a crust, then the outside is done cooking, even if the inside is not. Gently lay a piece of aluminum foil over the top of the cooking loaf to prevent the top from browning further.