Preheat oven to 375 degrees. Spray an 8x8 inch baking dish with canola oil or nonstick cooking spray. Unroll thawed phyllo dough and place it on your workspace. Remove one sheet, place a peeled banana along one end and sprinkle the rest of the dough with a fourth of the walnut powder. Sprinkle the banana with a little cinnamon and carefully roll it up in the phyllo dough. Place it seam-side down in the baking dish. Repeat with remaining bananas. Spray the top of the banans with canola oil. Bake for 20-30 minutes or until the tops are golden brown. In a small bowl, combine dark chocolate and vegetable oil. Microwave in 30 second increments until melted. Drizzle dark chocolate on top of baked bananas and sprinkle on any remaining walnuts.