Add the bananas, vanilla extract and Cold Brew coffee to a blender or food processor. Blend until smooth and creamy, scraping down the sides of the blender as necessary.
After 1-2 minutes of blending, add the cocoa powder and blend until well incorporated, scraping down the sides of the blender as necessary.
Pour into a freezer-friendly airtight container. Sprinkle chocolate chunks on top if desired, then freeze for at least 2 hours until the mixture is solid. Allow the ice cream to thaw for a 5-10 minutes before scooping.