Preheat oven to 350 F. Grease a cookie sheet or line with parchment paper and set aside. Add bulgar with 3/4 cup of water in a pot without heat and set aside to soak. Meanwhile, cook yam until fork tender by steaming, microwaving or boiling (peeling skin away is optional). Using hand beaters or a potato masher, blend cooked yam with non-dairy milk until the consistency is like mashed potatoes and set aside. Return to bulgar, which should have absorbed some of the water. Bring the pot to a boil and once boiling reduce to low heat and simmer. Continue to cook, uncovered, until all the water is gone, stirring frequently to prevent burning.
Meanwhile toss garlic, parsley, onion powder, cumin, coriander, chickpea flour, bread crumbs and salt in a food processor. Allow the motor run until it's very crumbly and spice-like. Mix spice mixture with yam mixture until well combined. Then add in cooked bulgar. Drop tablespoon-sized portion on to the cookie sheet and spray with cooking spray. Lightly cover with foil and bake 20 minutes. Flip the balls, respray and then bake another 20 minutes, or until the outsides are crispy.
While the falafel balls are baking, make cilantro-yogurt sauce. Stuff warm whole-wheat pitas with falafel balls, chopped lettuce and the "yogurt" sauce.