Peel the bananas and cut into slices. Freeze until hard, at least 2 hours or overnight.
Place the frozen bananas in a food processor fitted with the steel blade and process, scraping down the sides of the processor frequently, until smooth and creamy like soft serve ice cream, about 5 minutes. Pulse in the chopped chocolate and walnuts. Place in an airtight container, cover, and freeze for at least 1 hour.
If the ice cream is in the freezer for more than an hour it can get quite hard. If this happens, simply let it sit at room temperature for 5-10 minutes to soften before serving.