Melt butter or margarine with brown sugar and chocolate (about 2 minutes in a 1000 watt microwave should do the trick). Stir in bananas, eggs, flour, baking powder, and walnuts if desired. Pour into greased loaf pan and bake in a preheated oven at 300° F for one hour. Allow to cool in pan for 5 or 10 minutes before turning it onto a wire rack to finish cooling.
I sometimes use three bananas instead of two, and sometimes put some extra butter in as well. If this is done however, great care must be taken when turning the loaf out onto the wire rack. When it is warm, this bread is quite gooey, and a lot of the butter seems to rise to the top during baking leaving a buttery chocolate yummy mess in the middle that will slide right off if you aren't careful. Don't worry, it solidifies as it cools. I like it with walnuts, but most people feel that chocolate and bananas together are more than enough flavor. The basic recipe comes from one of my many cookbooks, but I can't remember which one just now.