Preheat the oven to 400F. Cut the bananas into about half inch pieces, and toss them in a small baking dish with the butter and the brown sugar. Bake the bananas for about 40 minutes, stirring them around once midway through. Set them aside and let them cool off a bit.
Once they’ve cooled, mix the bananas with the milk, sugar, vanilla, and salt in a blender or food processor and puree until you have a nice, smooth mixture. Let it cool in the refrigerator for at least an hour.
When the mixture is nice and cold, just process it according to your ice cream maker’s instructions. When the ice cream is nearly thickened to your liking, break the chocolate up into a microwave safe container, preferably one with a pour spout, like a pyrex measuring cup. Melt the chocolate at 10 or 15 second intervals, stirring between each interval, until it’s melted through and pourable. Slowly pour it into the almost finished ice cream; it will freeze and form small chips throughout the ice cream. Finally, add the chopped walnuts and continue to process until the walnuts are mixed through and the ice cream has reached your desired consistency.