Mash the bananas.
Beat the egg.
Mix the bananas, the milk, the egg and the butter (softened) on a bowl, and the rest of the ingredients on a separate one.
Add the dry mixture to the wet one little by little while mixing.
When everything is well mixed, pour it on a cake pan (pre-greased with butter) and put it in the oven (180°C) for half an hour, or until you stick a toothpick on it and it comes out clean. If it starts getting too brown for your liking on the top, place a foil sheet over it.
When the cake is fully cooked, let it cool while you melt a bar of white chocolate, and on a separate bowl, a bar of dark chocolate. I usually melt them with the bowl on a pot with hot water. Be careful not to let the water get in the chocolate.
When the cake is cold (or not too hot anymore), take it out of the pan carefully and pour the chocolate on top of it. Don't let the chocolate get cold before you pour it, or it will solidify. You can use a paper to divide the chocolates, or mix them just a little for a marble effect and pour it all on the cake. You can use a rack on top of the sink if you're worried about it getting messy.