Add 3/4 cup of chopped dark chocolate to a pot on the stove set to low heat (reserve the remaining chocolate for later).
Cook, stirring constantly until the chocolate has melted, then immediately remove the pot from the heat.
Add the melted dark chocolate and chopped banana to the food processor.
Blend until smooth, scraping down the sides of the food processor as needed.
Transfer the banana chocolate mixture to a bowl, then refrigerate it for 40 minutes or until it’s solid enough to hold its shape.
Scoop out 1 tablespoon of the chilled banana chocolate mixture and use your hands to shape it into a ball (I recommend wearing kitchen gloves to make this easy), then place it on a large plate lined with parchment paper or a silicone baking mat.
Repeat with the rest of the banana chocolate mixture (you should end up with 12 balls).
Add the remaining 3/4 cup of chopped chocolate to a pot on the stove set to low heat.
Cook, stirring constantly until the chocolate has melted, then immediately remove the pot from the heat.
Transfer the melted chocolate to a small bowl.
Add one of the banana chocolate balls to the bowl of melted dark chocolate, then gently stir to coat it completely.
Use a fork to lift the chocolate coated ball out of the bowl, allowing any excess chocolate to drip back into the bowl, then place it on the plate lined with parchment paper or a silicone baking mat.
Add a small sprinkle of finely chopped walnuts on top, if desired.
Repeat steps 11-13 with the rest of the banana chocolate balls.
Place the plate of truffles in the fridge to chill for at least 1 hour before serving.