Cook the 1 package Knorr Mexican or Spanish rice according to the package directions; keep warm.
Make the avocado ranch sauce: in a blender combine ½ cup sour cream, ⅓ cup buttermilk, ½ cup ripe avocado, 1 teaspoon chopped garlic, 2 teaspoons lime juice, and 2 teaspoons dry ranch dressing mix. Puree until smooth, transfer to a small airtight container, and refrigerate until assembly.
Chop the 1 tomato. Measure and set aside 1 teaspoon of the chopped tomato for the guacamole; reserve the remaining chopped tomato for topping.
Make the guacamole: mash the 2 small (or 1 large) avocado in a bowl. Add the reserved 1 teaspoon chopped tomato, 2 teaspoons lime juice, 1 tablespoon diced red onion, and ½ teaspoon salt. Stir gently to combine and set aside.
Drain and rinse the 1 (15-ounce) can black beans; keep warm or at room temperature until assembly.
Cook the chicken: pat the ½ pound chicken breast dry and season both sides with ½ teaspoon salt and ¼ teaspoon ground black pepper. Heat a skillet over medium heat and add 2 teaspoons olive oil. When the oil is hot, add the chicken and cook about 7 to 9 minutes per side (actual time will depend on thickness) until cooked through. Remove from the skillet, let rest about 3 minutes, then slice into thin strips.
Prepare remaining toppings: measure out ½ cup shredded Cheddar cheese, ½ cup low fat sour cream, and 2 cups shredded lettuce.
Assemble the Power Bowl(s): place the cooked rice in the bottom of each bowl (or one bowl). Top with sliced chicken, drained black beans, guacamole, the reserved chopped tomato, ½ cup low fat sour cream, ½ cup shredded Cheddar cheese, and 2 cups shredded lettuce (divide amounts as desired). Drizzle the avocado ranch sauce over the top before serving.