1. Cut the banana into small slices and freeze overnight.
2. Let banana slices thaw for ten minutes, then blend with coconut milk, agave syrup and vanilla extract until creamy. Add cacao and blend on a lower setting.
3. Place a scoop of the banana ice cream in the center of a chilled martini glass.
4. Add the rum off to the side of the ice cream, then pour the chilled coffee over it.