In a blender or food processor, combine the avocado, sour cream, lime juice, garlic powder, salt, and black pepper. Blend until smooth and creamy. Set aside.
Warm the corn tortillas in a skillet over medium heat for about 30 seconds on each side, or until pliable.
To assemble the tacos, place a generous amount of shredded chicken on each tortilla.
Top the chicken with shredded lettuce, diced tomatoes, and shredded cheddar cheese.
Drizzle the creamy avocado sauce over the top of each taco.
Garnish with chopped cilantro and serve immediately.