¼ cup (25 g) extra-dark chocolate bar, chopped into tiny pieces
Place the frozen banana slices, cacao powder and maple syrup in a high-speed blender or food processor and process until smooth. Occasionally scrape down the sides and continue to blend until smooth, approximately 3 to 5 minutes. Be careful, however, not to over-process, as the ice cream will become too liquid.
Add in the walnuts and chocolate pieces and blend for a few more seconds.
Serve immediately, or for firmer ice cream, place in an airtight, freezer-safe container and freeze for at least 1 hour or until solid.