In a large food processor fitted with an "S" blade, grind the walnuts into a fine meal. Add in the rest of the ingredients, and process again until a sticky, uniform dough is formed. Scoop the batter by heaping tablespoons onto a plate or baking sheet lined with parchment paper, to prevent sticking. Roll the balls between your hands, and roll them in cocoa powder, if desired.
Store the balls in the fridge or freezer, and serve them chilled for the most firm texture. (I prefer them frozen, myself.)