Puree 2 1/2 cups beans with cumin, chili powder and cayene pepper until smooth. Add rice, walnuts, onion, corn and remaining beans; pulse 2 or 3 times to mix (mixture should be stiff but not dry). Add 2 or 3 tablespoons water to moisten, if necessary. Season to taste with salt and pepper. Cover and chill if making ahead.
Divide mixture into 8 burgers. Dredge burgers in cornmeal. Chill 30 minutes (unless mixture has been made earlier and chilled).
Coat nonstick pan or grill with cooking spray. Cook burgers over medium heat 4 minutes. Flip, and cook 4 minutes more or until heated through. Garnish with guacamole and salsa, if desired, and serve.
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.